MORGAN STANLEY DEAN WITTER INTERMEDIATE INCOME SECURITIES
497, 1998-11-06
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<PAGE>
                         MORGAN STANLEY DEAN WITTER
                         INTERMEDIATE INCOME SECURITIES
                         PROSPECTUS--OCTOBER 30, 1998
 
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MORGAN STANLEY DEAN WITTER INTERMEDIATE INCOME SECURITIES (THE "FUND") IS AN
OPEN-END, DIVERSIFIED MANAGEMENT INVESTMENT COMPANY, WHOSE INVESTMENT OBJECTIVE
IS HIGH CURRENT INCOME CONSISTENT WITH SAFETY OF PRINCIPAL. THE FUND SEEKS TO
ACHIEVE ITS INVESTMENT OBJECTIVE BY INVESTING PRIMARILY IN INTERMEDIATE TERM,
INVESTMENT GRADE FIXED-INCOME SECURITIES. SEE "INVESTMENT OBJECTIVE AND
POLICIES."
 
The Fund offers four classes of shares (each, a "Class"), each with a different
combination of sales charges, ongoing fees and other features. The different
distribution arrangements permit an investor to choose the method of purchasing
shares that the investor believes is most beneficial given the amount of the
purchase, the length of time the investor expects to hold the shares and other
relevant circumstances. See "Purchase of Fund Shares--Alternative Purchase
Arrangements."
 
This Prospectus sets forth concisely the information you should know before
investing in the Fund. It should be read and retained for future reference.
Additional information about the Fund is contained in the Statement of
Additional Information, dated October 30, 1998, which has been filed with the
Securities and Exchange Commission, and which is available at no charge upon
request of the Fund at the address or telephone numbers listed on this page. The
Statement of Additional Information is incorporated herein by reference.
 
<TABLE>
<CAPTION>
TABLE OF CONTENTS
 
<S>                                                 <C>
Prospectus Summary................................       2
 
Summary of Fund Expenses..........................       4
 
Financial Highlights..............................       5
 
The Fund and its Management.......................       8
 
Investment Objective and Policies.................       8
 
  Risk Considerations.............................      11
 
Investment Restrictions...........................      11
 
Purchase of Fund Shares...........................      12
 
Shareholder Services..............................      20
 
Redemptions and Repurchases.......................      22
 
Dividends, Distributions and Taxes................      23
 
Performance Information...........................      24
 
Additional Information............................      24
</TABLE>
 
SHARES OF THE FUND ARE NOT DEPOSITS OR OBLIGATIONS OF, OR GUARANTEED OR ENDORSED
BY, ANY BANK, AND THE SHARES ARE NOT FEDERALLY INSURED BY THE FEDERAL DEPOSIT
INSURANCE CORPORATION, THE FEDERAL RESERVE BOARD, OR ANY OTHER AGENCY.
 
MORGAN STANLEY DEAN WITTER
INTERMEDIATE INCOME SECURITIES
TWO WORLD TRADE CENTER
NEW YORK, NEW YORK 10048
(212) 392-2550 or (800) 869-NEWS (toll-free)
 
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  THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
 EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS THE SECURITIES
   AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
                               CRIMINAL OFFENSE.
 
           MORGAN STANLEY DEAN WITTER DISTRIBUTORS INC., DISTRIBUTOR
<PAGE>
PROSPECTUS SUMMARY
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<TABLE>
<S>             <C>
THE FUND        The Fund is organized as a Trust, commonly known as a Massachusetts business trust, and
                is an open-end, diversified management investment company. The Fund invests primarily in
                intermediate term, investment grade fixed-income securities.
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SHARES OFFERED  Shares of beneficial interest with $0.01 par value (see page 24). The Fund offers four
                Classes of shares, each with a different combination of sales charges, ongoing fees and
                other features (see pages 12).
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MINIMUM         The minimum initial investment for each Class is $1,000 ($100 if the account is opened
PURCHASE        through EasyInvest-SM-). Class D shares are only available to persons investing $5
                million ($25 million for certain qualified plans) or more and to certain other limited
                categories of investors. For the purpose of meeting the minimum $5 million (or $25
                million) investment for Class D shares, and subject to the $1,000 minimum initial
                investment for each Class of the Fund, an investor's existing holdings of Class A shares
                and shares of funds for which Morgan Stanley Dean Witter Advisors Inc. serves as
                investment manager ("Morgan Stanley Dean Witter Funds") that are sold with a front-end
                sales charge, and concurrent investments in Class D shares of the Fund and other Morgan
                Stanley Dean Witter Funds that are multiple class funds, will be aggregated. The minimum
                subsequent investment is $100 (see page 12).
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INVESTMENT      The investment objective of the Fund is high current income consistent with safety of
OBJECTIVE       principal (see page 8).
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INVESTMENT      Morgan Stanley Dean Witter Advisors Inc., the Investment Manager of the Fund, and its
MANAGER         wholly-owned subsidiary, Morgan Stanley Dean Witter Services Company Inc., serve in
                various investment management, advisory, management and administrative capacities to 100
                investment companies and other portfolios with assets of approximately $113.7 billion at
                September 30, 1998 (see page 8).
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MANAGEMENT FEE  The Investment Manager receives a monthly fee at the annual rate of 0.60% of daily net
                assets, scaled down on assets over $500 million (see page 8).
- -------------------------------------------------------------------------------------------------------
DISTRIBUTOR     Morgan Stanley Dean Witter Distributors Inc. is the Distributor of the Fund's shares.
AND             The Fund has adopted a distribution plan pursuant to Rule 12b-1 under the Investment
DISTRIBUTION    Company Act (the "12b-1 Plan") with respect to the distribution fees paid by the Class
FEE             A, Class B and Class C shares of the Fund to the Distributor. The entire 12b-1 fee
                payable by Class A and a portion of the 12b-1 fee payable by each of Class B and Class C
                equal to 0.20% of the average daily net assets of Class B and 0.25% of the average daily
                net assets of Class C are currently each characterized as a service fee within the
                meaning of the National Association of Securities Dealers, Inc. guidelines. The
                remaining portion of the 12b-1 fee, if any, is characterized as an asset-based sales
                charge (see pages 12 and 18).
- -------------------------------------------------------------------------------------------------------
ALTERNATIVE     Four classes of shares are offered:
PURCHASE
ARRANGEMENTS    - Class A shares are offered with a front-end sales charge, starting at 4.25% and
                reduced for larger purchases. Investments of $1 million or more (and investments by
                certain other limited categories of investors) are not subject to any sales charge at
                the time of purchase but a contingent deferred sales charge ("CDSC") of 1.0% may be
                imposed on redemptions within one year of purchase. The Fund is authorized to reimburse
                the Distributor for specific expenses incurred in promoting the distribution of the
                Fund's Class A shares and servicing shareholder accounts pursuant to the Fund's 12b-1
                Plan. Reimbursement may in no event exceed an amount equal to payments at an annual rate
                of 0.25% of average daily net assets of the Class (see pages 12, 14 and 18).
 
                - Class B shares are offered without a front-end sales charge, but will in most cases be
                subject to a CDSC (scaled down from 5.0% to 1.0%) if redeemed within six years after
                purchase. The CDSC will be imposed on any redemption of shares if after such redemption
                the aggregate current value of a Class B account with the Fund falls below the aggregate
                amount of the investor's purchase payments made during the six years preceding the
                redemption. A different CDSC schedule applies to investments by certain qualified plans.
                Class B shares are also subject to a 12b-1 fee assessed at the annual rate of 0.85% of
                the lesser of: (a) the average daily net sales of the Fund's Class B shares or (b) the
                average daily net assets of Class B. All shares of the Fund held prior to July 28, 1997
                (other than the shares held by certain employee benefit plans established by certain
                subsidiaries of Morgan Stanley Dean Witter & Co.) have been designated Class B shares.
                Shares held by those employee benefit plans prior to July 28, 1997 have been designated
                Class D shares. Shares held before May 1, 1997 that have been designated Class B shares
                will convert to Class A shares in May 2007. In all other instances, Class B shares
                convert to Class A shares approximately ten years after the date of the original
                purchase (see pages 12, 16 and 18).
</TABLE>
 
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2
<PAGE>
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<TABLE>
<S>             <C>
                - Class C shares are offered without a front-end sales charge, but will in most cases be
                subject to a CDSC of 1.0% if redeemed within one year after purchase. The Fund is
                authorized to reimburse the Distributor for specific expenses incurred in promoting the
                distribution of the Fund's Class C shares and servicing shareholder accounts pursuant to
                the Fund's 12b-1 Plan. Reimbursement may in no event exceed an amount equal to payments
                at an annual rate of 0.85% of average daily net assets of the Class (see pages 12 and
                18).
 
                - Class D shares are offered only to investors meeting an initial investment minimum of
                $5 million ($25 million for certain qualified plans) and to certain other limited
                categories of investors. Class D shares are offered without a front-end sales charge or
                CDSC and are not subject to any 12b-1 fee (see pages 12 and 18).
- -------------------------------------------------------------------------------------------------------
DIVIDENDS AND   Dividends are declared daily, and either paid monthly in additional shares of the Fund
CAPITAL GAINS   or, at the shareholder's option, paid monthly in cash. Dividends from net short-term,
DISTRIBUTIONS   mid-term and long-term capital gains, if any, are distributed at least once each year.
                The Fund may, however, determine to retain all or part of any net long-term and net
                mid-term capital gains in any year for reinvestment. Dividends and capital gains
                distributions paid on shares of a Class are automatically reinvested in additional
                shares of the same Class at net asset value unless the shareholder elects to receive
                cash. Shares acquired by dividend and distribution reinvestment will not be subject to
                any sales charge or CDSC (see pages 20 and 23).
- -------------------------------------------------------------------------------------------------------
REDEMPTION      Shares are redeemable by the shareholder at net asset value less any applicable CDSC on
                Class A, Class B or Class C shares. An account may be involuntarily redeemed if the
                total value of the account is less than $100 or, if the account was opened through
                EasyInvest-SM-, if after twelve months the shareholder has invested less than $1,000 in
                the account (see page 22).
- -------------------------------------------------------------------------------------------------------
RISK            The net asset value of the Fund's shares will fluctuate with changes in the market value
CONSIDERATIONS  of its portfolio securities. Interest rate fluctuations will affect the Fund's net asset
                value but not the income received by the Fund from its portfolio securities. The Fund
                may engage in various investment strategies including reverse repurchase agreements,
                when-issued and delayed delivery securities and forward commitments and when, as and if
                issued securities. The risks associated with these investments are included in their
                description (pages 9-12) and in the "Risk Considerations" section (page 11).
</TABLE>
 
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  THE ABOVE IS QUALIFIED IN ITS ENTIRETY BY THE DETAILED INFORMATION APPEARING
              ELSEWHERE IN THIS PROSPECTUS AND IN THE STATEMENT OF
                            ADDITIONAL INFORMATION.
 
                                                                               3
<PAGE>
SUMMARY OF FUND EXPENSES
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The following table illustrates all expenses and fees that a shareholder of the
Fund will incur. The expenses and fees set forth in the table are based on the
expenses and fees for the fiscal year ended August 31, 1998.
 
<TABLE>
<CAPTION>
                                                                                   CLASS A    CLASS B    CLASS C    CLASS D
                                                                                  ---------   -------   ---------   -------
<S>                                                                               <C>         <C>       <C>         <C>
SHAREHOLDER TRANSACTION EXPENSES
Maximum Sales Charge Imposed on Purchases (as a percentage of offering price)...   4.25%(1)    None      None        None
Sales Charge Imposed on Dividend Reinvestments..................................   None        None      None        None
Maximum Contingent Deferred Sales Charge (as a percentage of original purchase
 price or redemption proceeds)..................................................   None(2)    5.00%(3)   1.00%(4)    None
Redemption Fees.................................................................   None        None      None        None
Exchange Fee....................................................................   None        None      None        None
 
ANNUAL FUND OPERATING EXPENSES (AS A PERCENTAGE OF AVERAGE NET ASSETS)
Management Fees.................................................................      0.60%     0.60%       0.60%     0.60%
12b-1 Fees (5) (6)..............................................................      0.24%     0.85%       0.85%    None
Other Expenses..................................................................      0.26%     0.26%       0.26%     0.26%
Total Fund Operating Expenses...................................................      1.10%     1.71%       1.71%     0.86%
</TABLE>
 
- ---------------
(1) REDUCED FOR PURCHASES OF $25,000 AND OVER (SEE "PURCHASE OF FUND
    SHARES--INITIAL SALES CHARGE ALTERNATIVE--CLASS A SHARES").
(2) INVESTMENTS THAT ARE NOT SUBJECT TO ANY SALES CHARGE AT THE TIME OF PURCHASE
    ARE SUBJECT TO A CDSC OF 1.00% THAT WILL BE IMPOSED ON REDEMPTIONS MADE
    WITHIN ONE YEAR AFTER PURCHASE, EXCEPT FOR CERTAIN SPECIFIC CIRCUMSTANCES
    (SEE "PURCHASE OF FUND SHARES--INITIAL SALES CHARGE ALTERNATIVE--CLASS A
    SHARES").
(3) THE CDSC IS SCALED DOWN TO 1.00% DURING THE SIXTH YEAR, REACHING ZERO
    THEREAFTER.
(4) ONLY APPLICABLE TO REDEMPTIONS MADE WITHIN ONE YEAR AFTER PURCHASE (SEE
    "PURCHASE OF FUND SHARES--LEVEL LOAD ALTERNATIVE--CLASS C SHARES").
(5) THE 12B-1 FEE IS ACCRUED DAILY AND PAYABLE MONTHLY. THE ENTIRE 12B-1 FEE
    PAYABLE BY CLASS A AND A PORTION OF THE 12B-1 FEE PAYABLE BY EACH OF CLASS B
    AND CLASS C EQUAL TO 0.20% OF THE AVERAGE DAILY NET ASSETS OF CLASS B AND
    0.25% OF THE AVERAGE DAILY NET ASSETS OF CLASS C ARE CURRENTLY EACH
    CHARACTERIZED AS A SERVICE FEE WITHIN THE MEANING OF NATIONAL ASSOCIATION OF
    SECURITIES DEALERS, INC. ("NASD") GUIDELINES AND ARE PAYMENTS MADE FOR
    PERSONAL SERVICE AND/OR MAINTENANCE OF SHAREHOLDER ACCOUNTS. THE REMAINDER
    OF THE 12B-1 FEE, IF ANY, IS AN ASSET-BASED SALES CHARGE, AND IS A
    DISTRIBUTION FEE PAID TO THE DISTRIBUTOR TO COMPENSATE IT FOR THE SERVICES
    PROVIDED AND THE EXPENSES BORNE BY THE DISTRIBUTOR AND OTHERS IN THE
    DISTRIBUTION OF THE FUND'S SHARES (SEE "PURCHASE OF FUND SHARES--PLAN OF
    DISTRIBUTION").
(6) UPON CONVERSION OF CLASS B SHARES TO CLASS A SHARES, SUCH SHARES WILL BE
    SUBJECT TO THE LOWER 12B-1 FEE APPLICABLE TO CLASS A SHARES. NO SALES CHARGE
    IS IMPOSED AT THE TIME OF CONVERSION OF CLASS B SHARES TO CLASS A SHARES.
    CLASS C SHARES DO NOT HAVE A CONVERSION FEATURE AND, THEREFORE, ARE SUBJECT
    TO AN ONGOING 0.85% DISTRIBUTION FEE (SEE "PURCHASE OF FUND
    SHARES--ALTERNATIVE PURCHASE ARRANGEMENTS").
 
<TABLE>
<CAPTION>
EXAMPLES                                            1 YEAR   3 YEARS   5 YEARS   10 YEARS
                                                    ------   -------   -------   --------
<S>                                                 <C>      <C>       <C>       <C>
You would pay the following expenses on a $1,000
 investment assuming (1) a 5% annual return and
 (2) redemption at the end of each time period:
    Class A.......................................   $53       $76       $100      $171
    Class B.......................................   $67       $84       $113      $202
    Class C.......................................   $27       $54       $93       $202
    Class D.......................................   $ 9       $27       $48       $106
You would pay the following expenses on the same
 $1,000 investment assuming no redemption at the
 end of the period:
    Class A.......................................   $53       $76       $100      $171
    Class B.......................................   $17       $54       $93       $202
    Class C.......................................   $17       $54       $93       $202
    Class D.......................................   $ 9       $27       $48       $106
</TABLE>
 
THE ABOVE EXAMPLES SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR FUTURE
EXPENSES OR PERFORMANCE. ACTUAL EXPENSES OF EACH CLASS MAY BE GREATER OR LESS
THAN THOSE SHOWN.
 
The purpose of this table is to assist the investor in understanding the various
costs and expenses that an investor in the Fund will bear directly or
indirectly. For a more complete description of these costs and expenses, see
"The Fund and Its Management," "Purchase of Fund Shares--Plan of Distribution"
and "Redemptions and Repurchases."
 
Long-term shareholders of Class B and Class C may pay more in sales charges,
including distribution fees, than the economic equivalent of the maximum
front-end sales charges permitted by the NASD.
 
4
<PAGE>
FINANCIAL HIGHLIGHTS
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The following ratios and per share data for a share of beneficial interest
outstanding throughout each period have been audited by PricewaterhouseCoopers
LLP, independent accountants. The financial highlights should be read in
conjunction with the financial statements, notes thereto, and the unqualified
report of independent accountants, which are contained in the Statement of
Additional Information. Further information about the performance of the Fund is
contained in the Fund's Annual Report to Shareholders, which may be obtained
without charge upon request to the Fund.
<TABLE>
<CAPTION>
                                        FOR THE YEAR ENDED AUGUST 31
                             ---------------------------------------------------
CLASS B SHARES               1998++     1997**++    1996       1995       1994
                             -------    -------    -------    -------    -------
 
<S>                          <C>        <C>        <C>        <C>        <C>
PER SHARE OPERATING
 PERFORMANCE:
Net asset value, beginning
 of period................   $  9.59    $  9.39    $  9.69    $  9.51    $ 10.26
                             -------    -------    -------    -------    -------
Net investment income.....      0.51       0.53       0.55       0.59       0.58
Net realized and
 unrealized gain (loss)...      0.11       0.20      (0.30)      0.19      (0.73)
                             -------    -------    -------    -------    -------
Total from investment
 operations...............      0.62       0.73       0.25       0.78      (0.15)
                             -------    -------    -------    -------    -------
Less dividends and
 distributions from:
  Net investment income...     (0.51)     (0.53)     (0.55)     (0.59)     (0.56)
  Net realized gain.......     --         --         --         (0.01)     (0.04)
                             -------    -------    -------    -------    -------
Total dividends and
 distributions............     (0.51)     (0.53)     (0.55)     (0.60)     (0.60)
                             -------    -------    -------    -------    -------
Net asset value, end of
 period...................   $  9.70    $  9.59    $  9.39    $  9.69    $  9.51
                             -------    -------    -------    -------    -------
                             -------    -------    -------    -------    -------
 
TOTAL INVESTMENT
 RETURN+..................     6.53%      7.93%      2.58%      8.56%    (1.50)%
 
RATIOS TO AVERAGE NET
 ASSETS:
Expenses..................     1.71%(4)   1.65%      1.62%      1.63%      1.63%
Net investment income.....     5.19%(4)   5.52%      5.69%      6.23%      5.80%
 
SUPPLEMENTAL DATA:
Net assets, end of period,
 in thousands.............   $151,515   $162,959   $208,911   $232,752   $245,750
Portfolio turnover rate...       98%        98%       115%       114%       122%
 
<CAPTION>
                                                                         FOR THE PERIOD
                                                                          MAY 3, 1989*
                                                                             THROUGH
CLASS B SHARES                1993       1992       1991       1990      AUGUST 31, 1989
                             -------    -------    -------    -------    ---------------
<S>                          <C>        <C>        <C>        <C>        <C>
PER SHARE OPERATING
 PERFORMANCE:
Net asset value, beginning
 of period................   $ 10.05    $  9.59    $  9.42    $  9.98    $   10.00
                             -------    -------    -------    -------       ------
Net investment income.....      0.62       0.70       0.79       0.86         0.28
Net realized and
 unrealized gain (loss)...      0.20       0.46       0.17      (0.55)       (0.02)
                             -------    -------    -------    -------       ------
Total from investment
 operations...............      0.82       1.16       0.96       0.31         0.26
                             -------    -------    -------    -------       ------
Less dividends and
 distributions from:
  Net investment income...     (0.61)     (0.70)     (0.79)     (0.86)       (0.28)
  Net realized gain.......     --         --         --         (0.01)      --
                             -------    -------    -------    -------       ------
Total dividends and
 distributions............     (0.61)     (0.70)     (0.79)     (0.87)       (0.28)
                             -------    -------    -------    -------       ------
Net asset value, end of
 period...................   $ 10.26    $ 10.05    $  9.59    $  9.42    $    9.98
                             -------    -------    -------    -------       ------
                             -------    -------    -------    -------       ------
TOTAL INVESTMENT
 RETURN+..................     8.43%     12.58%     10.78%      3.22%        2.57%(1)
RATIOS TO AVERAGE NET
 ASSETS:
Expenses..................     1.62%      1.69%      1.69%      1.75%        1.42%(2)(3)
Net investment income.....     6.12%      7.11%      8.49%      8.78%        8.18%(2)(3)
SUPPLEMENTAL DATA:
Net assets, end of period,
 in thousands.............   $254,431   $187,285   $115,204   $114,086     $69,946
Portfolio turnover rate...      132%        93%       150%       135%          30%(1)
</TABLE>
 
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 * COMMENCEMENT OF OPERATIONS.
 
 ** PRIOR TO JULY 28, 1997, THE FUND ISSUED ONE CLASS OF SHARES. ALL SHARES OF
    THE FUND HELD PRIOR TO THAT DATE, OTHER THAN SHARES HELD BY CERTAIN EMPLOYEE
    BENEFIT PLANS ESTABLISHED BY DEAN WITTER REYNOLDS INC. AND ITS AFFILIATE,
    SPS TRANSACTION SERVICES, INC., HAVE BEEN DESIGNATED AS CLASS B SHARES.
    SHARES HELD BY THOSE EMPLOYEE BENEFIT PLANS PRIOR TO JULY 28, 1997 HAVE BEEN
    DESIGNATED CLASS D SHARES.
 
 ++ THE PER SHARE AMOUNTS WERE COMPUTED USING AN AVERAGE NUMBER OF SHARES
    OUTSTANDING DURING THE PERIOD.
 
 + DOES NOT REFLECT THE DEDUCTION OF SALES CHARGE. CALCULATED BASED ON THE NET
   ASSET VALUE AS OF THE LAST BUSINESS DAY OF THE PERIOD.
 
(1) NOT ANNUALIZED.
 
(2) ANNUALIZED.
 
(3) IF THE FUND HAD BORNE ALL THE EXPENSES THAT WERE ASSUMED OR WAIVED BY THE
    INVESTMENT MANAGER, THE ABOVE EXPENSE AND NET INVESTMENT INCOME RATIOS WOULD
    HAVE BEEN 2.15% AND 7.44%, RESPECTIVELY.
 
(4) REFLECTS OVERALL FUND RATIOS FOR INVESTMENT INCOME AND NON-CLASS SPECIFIC
EXPENSES.
 
                                                                               5
<PAGE>
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<TABLE>
<CAPTION>
                                                                                            FOR THE PERIOD
                                                                          FOR THE YEAR      JULY 28, 1997*
                                                                             ENDED             THROUGH
                                                                           AUGUST 31,         AUGUST 31,
CLASS A SHARES                                                               1998++             1997++
                                                                        ----------------   ----------------
<S>                                                                     <C>                <C>
PER SHARE OPERATING PERFORMANCE:
Net asset value, beginning of period..................................      $  9.59            $  9.68
                                                                             ------             ------
Net investment income.................................................         0.56               0.06
Net realized and unrealized gain (loss)...............................         0.13              (0.10)
                                                                             ------             ------
Total from investment operations......................................         0.69              (0.04)
                                                                             ------             ------
Less dividends from net investment income.............................        (0.57)             (0.05)
                                                                             ------             ------
Net asset value, end of period........................................      $  9.71            $  9.59
                                                                             ------             ------
                                                                             ------             ------
TOTAL INVESTMENT RETURN+..............................................         7.30%             (0.46)%(1)
RATIOS TO AVERAGE NET ASSETS:
Expenses..............................................................         1.10%(3)           2.18%(2)
Net investment income.................................................         5.80%(3)           6.10%(2)
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands...............................       $3,457             $1,855
Portfolio turnover rate...............................................           98%                98%
 
<CAPTION>
 
CLASS C SHARES
<S>                                                                     <C>                <C>
PER SHARE OPERATING PERFORMANCE:
Net asset value, beginning of period..................................      $  9.61            $  9.68
                                                                             ------             ------
Net investment income.................................................         0.49               0.04
Net realized and unrealized gain (loss)...............................         0.11              (0.07)
                                                                             ------             ------
Total from investment operations......................................         0.60              (0.03)
                                                                             ------             ------
Less dividends from net investment income.............................        (0.50)             (0.04)
                                                                             ------             ------
Net asset value, end of period........................................      $  9.71            $  9.61
                                                                             ------             ------
                                                                             ------             ------
TOTAL INVESTMENT RETURN+..............................................         6.39%             (0.31)%(1)
RATIOS TO AVERAGE NET ASSETS:
Expenses..............................................................         1.71%(3)           2.02%(2)
Net investment income.................................................         5.19%(3)           4.22%(2)
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands...............................         $457                $38
Portfolio turnover rate...............................................           98%                98%
</TABLE>
 
- -------------
 * THE DATE SHARES WERE FIRST ISSUED.
 
 ++ THE PER SHARE AMOUNTS WERE COMPUTED USING AN AVERAGE NUMBER OF SHARES
    OUTSTANDING DURING THE PERIOD.
 
 + DOES NOT REFLECT THE DEDUCTION OF SALES CHARGE. CALCULATED BASED ON THE NET
   ASSET VALUE AS OF THE LAST BUSINESS DAY OF THE PERIOD.
 
(1) NOT ANNUALIZED.
 
(2) ANNUALIZED.
 
(3) REFLECTS OVERALL FUND RATIOS FOR INVESTMENT INCOME AND NON-CLASS SPECIFIC
    EXPENSES.
 
6
<PAGE>
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<TABLE>
<CAPTION>
                                                                                            FOR THE PERIOD
                                                                                            JULY 28, 1997*
                                                                          FOR THE YEAR         THROUGH
                                                                              ENDED           AUGUST 31,
CLASS D SHARES                                                          AUGUST 31, 1998++       1997++
                                                                        -----------------   --------------
<S>                                                                     <C>                 <C>
PER SHARE OPERATING PERFORMANCE:
Net asset value, beginning of period..................................       $ 9.59             $ 9.68
                                                                             ------             ------
Net investment income.................................................         0.59               0.05
Net realized and unrealized gain (loss)...............................         0.11              (0.09)
                                                                             ------             ------
Total from investment operations......................................         0.70              (0.04)
                                                                             ------             ------
Less dividends from net investment income.............................        (0.59)             (0.05)
                                                                             ------             ------
Net asset value, end of period........................................       $ 9.70             $ 9.59
                                                                             ------             ------
                                                                             ------             ------
TOTAL INVESTMENT RETURN+..............................................         7.43%             (0.44)%(1)
 
RATIOS TO AVERAGE NET ASSETS:
Expenses..............................................................         0.86%(3)           1.11%(2)
Net investment income.................................................         6.04%(3)           5.91%(2)
 
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands...............................       $5,726             $4,880
Portfolio turnover rate...............................................           98%                98%
</TABLE>
 
- -------------
 * THE DATE SHARES WERE FIRST ISSUED.
 
 ++ THE PER SHARE AMOUNTS WERE COMPUTED USING AN AVERAGE NUMBER OF SHARES
    OUTSTANDING DURING THE PERIOD.
 
 + CALCULATED BASED ON THE NET ASSET VALUE AS OF THE LAST BUSINESS DAY OF THE
PERIOD.
 
(1) NOT ANNUALIZED.
 
(2) ANNUALIZED.
 
(3) REFLECTS OVERALL FUND RATIOS FOR INVESTMENT INCOME AND NON-CLASS SPECIFIC
    EXPENSES.
 
                                                                               7
<PAGE>
THE FUND AND ITS MANAGEMENT
- --------------------------------------------------------------------------------
 
Morgan Stanley Dean Witter Intermediate Income Securities (formerly named Dean
Witter Intermediate Income Securities) (the "Fund") is an open-end, diversified
management investment company. The Fund is a trust of the type commonly known as
a "Massachusetts business trust" and was organized under the laws of the
Commonwealth of Massachusetts on September 1, 1988.
 
    Morgan Stanley Dean Witter Advisors Inc. ("MSDW Advisors" or the "Investment
Manager"), whose address is Two World Trade Center, New York, New York 10048, is
the Fund's Investment Manager. The Investment Manager is a wholly-owned
subsidiary of Morgan Stanley Dean Witter & Co., a preeminent global financial
services firm that maintains leading market positions in each of its three
primary businesses--securities, asset management and credit services. The
Investment Manager, which was incorporated in July, 1992 under the name Dean
Witter InterCapital Inc., changed its name to Morgan Stanley Dean Witter
Advisors Inc. on June 22, 1998.
 
    MSDW Advisors and its wholly-owned subsidiary, Morgan Stanley Dean Witter
Services Company Inc. ("MSDW Services"), serve in various investment management,
advisory, management and administrative capacities to 100 investment companies,
28 of which are listed on the New York Stock Exchange, with combined assets of
approximately $109.6 billion at September 30, 1998. The Investment Manager also
manages portfolios of pension plans, other institutions and individuals which
aggregated approximately $4.1 billion at such date.
 
    The Fund has retained the Investment Manager to provide administrative
services, manage its business affairs and manage the investment of the Fund's
assets, including the placing of orders for the purchase and sale of portfolio
securities. MSDW Advisors has retained MSDW Services to provide the
aforementioned administrative services to the Fund. The Fund's Trustees review
the various services provided by or under the direction of the Investment
Manager to ensure that the Fund's general investment policies and programs are
being properly carried out and that administrative services are being provided
to the Fund in a satisfactory manner.
 
    As full compensation for the services and facilities furnished to the Fund
and for expenses of the Fund assumed by the Investment Manager, the Fund pays
the Investment Manager monthly compensation calculated daily by applying the
annual rate of 0.60% to the Fund's daily net assets up to $500 million, scaled
down at various levels to 0.30% on assets over $1 billion. For the fiscal year
ended August 31, 1998, the Fund accrued total compensation to the Investment
Manager amounting to 0.60% of the Fund's average daily net assets and the total
expenses of each Class amounted to 1.10%, 1.71%, 1.71% and 0.86% of the average
daily net assets of Class A, Class B, Class C and Class D, respectively.
 
INVESTMENT OBJECTIVE AND POLICIES
- --------------------------------------------------------------------------------
 
The investment objective of the Fund is high current income consistent with
safety of principal. There is no assurance that the objective will be achieved.
This investment objective is a fundamental policy and may not be changed without
approval of the Fund's shareholders. The Fund seeks to achieve its objective by
investing at least 65% of its total assets in intermediate term, investment
grade fixed-income securities. The Fund will maintain an average weighted
maturity of approximately seven years or less and may not invest in securities
with remaining maturities greater than twelve years. Under normal conditions,
the Fund's average weighted maturity will not be less than three years. (Under
the current interpretation by the staff of the Securities and Exchange
Commission, an intermediate bond fund must have an average weighted maturity
between three and ten years.)
 
    Under normal circumstances, the Fund will invest primarily in corporate debt
securities and preferred stock of investment grade, which consists of securities
which are rated at the time of purchase Baa or better by Moody's Investors
Service, Inc. ("Moody's") or BBB or better by Standard & Poor's Corporation
("Standard & Poor's"), or which, if unrated, are deemed to be of comparable
quality by the Fund's Trustees. Fixed-income securities rated Baa by Moody's
have speculative characteristics. (A more detailed description of bond ratings
is contained in the Appendix to the Statement of Additional Information.) The
Fund may also purchase U.S. Government securities (securities guaranteed as to
principal and interest by the United States or its agencies or
instrumentalities) and investment grade securities, denominated in U.S. Dollars,
issued by foreign governments or issuers. U.S. Government securities in which
the Fund may invest include zero coupon securities and mortgage-backed
securities, such as securities issued by the Government National Mortgage
Association, the Federal National Mortgage Association and the Federal Home Loan
Mortgage Corporation. There can be no assurance that the investment objective of
the Fund will be achieved.
 
    The Investment Manager believes that the Fund's policies of purchasing
intermediate term securities will reduce the volatility of the Fund's net asset
value over the long term. Although the values of fixed-income securities
generally increase during periods of declining interest rates and decrease
during periods of increasing interest rates, the extent of these fluctuations
has historically generally been smaller for intermediate term securities than
for securities with longer maturities. Conversely, the yield
 
8
<PAGE>
available on intermediate term securities has also historically been lower than
those available from long term securities.
 
    Investment by the Fund in U.S. Dollar denominated fixed-income securities
issued by foreign governments and other foreign issuers may involve certain
risks not associated with U.S. issued securities. Those risks include the
political or economic instability of the issuer or of the country of issue, the
difficulty of predicting international trade patterns and the possibility of
imposition of exchange controls. In addition, there may be less publicly
available information about a foreign company than about a domestic company. A
more detailed description of the general risks of foreign issuers is contained
in the Statement of Additional Information. The Fund believes that those risks
are substantially lessened because the foreign securities in which the Fund may
invest are investment grade.
 
    While the Fund will invest primarily in investment grade fixed-income
securities, under ordinary circumstances it also may invest up to 35% of its
total assets in money market instruments, repurchase agreements, as discussed
below, and up to 5% of the Fund's net assets may be invested in lower rated
fixed-income securities.
 
    Lower rated fixed-income securities, which are those rated from Ba to C or
BB to C by Moody's or Standard & Poor's, respectively, are considered to be
speculative investments. Such lower rated securities, while producing higher
yield than investment grade securities, are subject to credit risk to a greater
extent than investment grade securities. The Fund does not have any minimum
quality rating standard with respect to the portion (up to 5%) of its net assets
which may be invested in lower rated securities. See the Statement of Additional
Information for a description of the special risks and characteristics of lower
rated fixed-income securities.
 
    There may be periods during which, in the opinion of the Investment Manager,
market conditions warrant reduction of some or all of the Fund's securities
holdings. During such periods, the Fund may adopt a temporary "defensive"
posture in which greater than 35% of its total assets are invested in cash or
money market instruments. Money market instruments in which the Fund may invest
are securities issued or guaranteed by the U.S. Government (Treasury bills,
notes and bonds, including zero coupon securities); bank obligations; Eurodollar
certificates of deposit; obligations of savings institutions; fully insured
certificates of deposit; and commercial paper rated within the two highest
grades by Moody's or Standard & Poor's or, if not rated, are issued by a company
having an outstanding debt issue rated at least AA by Standard & Poor's or Aa by
Moody's.
 
ZERO COUPON SECURITIES.  A portion of the fixed-income securities purchased by
the Fund may be zero coupon securities. Such securities are purchased at a
discount from their face amount, giving the purchaser the right to receive their
full value at maturity. The interest earned on such securities is, implicitly,
automatically compounded and paid out at maturity. While such compounding at a
constant rate eliminates the risk of receiving lower yields upon reinvestment of
interest if prevailing interest rates decline, the owner of a zero coupon
security will be unable to participate in higher yields upon reinvestment of
interest received on interest-paying securities if prevailing interest rates
rise.
 
    A zero coupon security pays no interest to its holder during its life.
Therefore, to the extent the Fund invests in zero coupon securities, it will not
receive current cash available for distribution to shareholders. In addition,
zero coupon securities are subject to substantially greater price fluctuations
during periods of changing prevailing interest rates than are comparable
securities which pay interest on a current basis. Current federal tax law
requires that a holder (such as the Fund) of a zero coupon security accrue a
portion of the discount at which the security was purchased as income each year
even though the Fund receives no interest payments in cash on the security
during the year.
 
REPURCHASE AGREEMENTS.  The Fund may enter into repurchase agreements, which may
be viewed as a type of secured lending by the Fund, and which typically involve
the acquisition by the Fund of debt securities from a selling financial
institution such as a bank, savings and loan association or broker-dealer. The
agreement provides that the Fund will sell back to the institution, and that the
institution will repurchase, the underlying security at a specified price and at
a fixed time in the future, usually not more than seven days from the date of
purchase. While repurchase agreements involve certain risks not associated with
direct investments in debt securities, including the risks of default or
bankruptcy of the selling financial institution, the Fund follows procedures
designed to minimize those risks. These procedures include effecting repurchase
transactions only with large, well-capitalized and well established financial
institutions and maintaining adequate collateralization.
 
REVERSE REPURCHASE AGREEMENTS.  The Fund may also use reverse repurchase
agreements for purposes of meeting redemptions or as part of its investment
strategy. Reverse repurchase agreements involve sales by the Fund of portfolio
assets concurrently with an agreement by the Fund to repurchase the same assets
at a later date at a fixed price. Generally, the effect of such a transaction is
that the Fund can recover all or most of the cash invested in the portfolio
securities involved during the term of the reverse repurchase agreement, while
it will be able to keep the interest income associated with those portfolio
securities. Such transactions are only advantageous if the interest cost to the
Fund of the reverse repurchase transaction is less than the cost of obtaining
the cash otherwise. Opportunities to achieve this advantage may not always be
available, and the Fund intends to use the reverse repurchase technique only
when it will be to its advantage to do so. The Fund will establish a segregated
account with its custodian bank in
 
                                                                               9
<PAGE>
which it will maintain cash, U.S. Government securities or other liquid
portfolio securities equal in value to its obligations in respect of reverse
repurchase agreements. Reverse repurchase agreements are considered borrowings
by the Fund. The use of borrowed funds for other than emergency purposes
constitutes leveraging, which is a speculative technique. Reverse repurchase
agreements may not exceed 10% of the Fund's total assets.
 
WHEN-ISSUED AND DELAYED DELIVERY SECURITIES AND FORWARD COMMITMENTS.  From time
to time, in the ordinary course of business, the Fund may purchase securities on
a when-issued or delayed delivery basis or may purchase or sell securities on a
forward commitment basis. When such transactions are negotiated, the price is
fixed at the time of the commitment, but delivery and payment can take place a
month or more after the date of the commitment. There is no overall limit on the
percentage of the Fund's assets which may be committed to the purchase of
securities on a when-issued, delayed delivery or forward commitment basis. An
increase in the percentage of the Fund's assets committed to the purchase of
securities on a when-issued, delayed delivery or forward commitment basis may
increase the volatility of the Fund's net asset value.
 
WHEN, AS AND IF ISSUED SECURITIES.  The Fund may purchase securities on a "when,
as and if issued" basis under which the issuance of the security depends upon
the occurrence of a subsequent event, such as approval of a merger, corporate
reorganization, leveraged buyout or debt restructuring. If the anticipated event
does not occur and the securities are not issued, the Fund will have lost an
investment opportunity. There is no overall limit on the percentage of the
Fund's assets which may be committed to the purchase of securities on a "when,
as and if issued" basis. An increase in the percentage of the Fund's assets
committed to the purchase of securities on a "when, as and if issued" basis may
increase the volatility of its net asset value.
 
PRIVATE PLACEMENTS.  The Fund may invest up to 5% of its total assets in
securities which are subject to restrictions on resale because they have not
been registered under the Securities Act of 1933, as amended (the "Securities
Act"), or which are otherwise not readily marketable. (Securities eligible for
resale pursuant to Rule 144A under the Securities Act, and determined to be
liquid pursuant to the procedures discussed in the following paragraph, are not
subject to the foregoing restriction.) These securities are generally referred
to as private placements or restricted securities. Limitations on the resale of
such securities may have an adverse effect on their marketability, and may
prevent the Fund from disposing of them promptly at reasonable prices. The Fund
may have to bear the expense of registering such securities for resale and the
risk of substantial delays in effecting such registration.
 
    The Securities and Exchange Commission has adopted Rule 144A under the
Securities Act, which permits the Fund to sell restricted securities to
qualified institutional buyers without limitation. The Investment Manager,
pursuant to procedures adopted by the Trustees of the Fund, will make a
determination as to the liquidity of each restricted security purchased by the
Fund. If a restricted security is determined to be "liquid," such security will
not be included within the category "illiquid securities," which is limited by
the Fund's investment restrictions to 10% of the Fund's total assets. Investing
in Rule 144A securities could have the effect of increasing the level of Fund
illiquidity to the extent the Fund, at a particular point in time, may be unable
to find qualified institutional buyers interested in purchasing such securities.
 
LENDING OF PORTFOLIO SECURITIES.  Consistent with applicable regulatory
requirements, the Fund may lend its portfolio securities to brokers, dealers and
other financial institutions, provided that such loans are callable at any time
by the Fund (subject to certain notice provisions described in the Statement of
Additional Information), and are at all times secured by cash or cash
equivalents, which are maintained in a segregated account pursuant to applicable
regulations and that are at least equal to the market value, determined daily,
of the loaned securities.
 
PORTFOLIO MANAGEMENT
 
The Fund's portfolio is actively managed by its Investment Manager with a view
to achieving the Fund's investment objective. In determining which securities to
purchase for the Fund or hold in the Fund's portfolio, the Investment Manager
will rely on information from various sources, including research, analysis and
appraisals of brokers and dealers; the views of Trustees of the Fund and others
regarding economic developments and interest rate trends; and the Investment
Manager's own analysis of factors it deems relevant. The Fund's portfolio is
managed within MSDW Advisors' Taxable Income Group, which manages twenty-one
funds and fund portfolios, with approximately $14.1 billion in assets as of
September 30, 1998. Rochelle G. Siegel, Senior Vice President of MSDW Advisors
and a member of MSDW Advisors' Taxable Group, has been the primary portfolio
manager since the Fund's inception and has been a portfolio manager at MSDW
Advisors since July, 1985.
 
    Pursuant to an order of the Securities and Exchange Commission, the Fund may
effect principal transactions in certain money market instruments with Dean
Witter Reynolds Inc. In addition, the Fund may incur brokerage commissions on
transactions conducted through Dean Witter Reynolds Inc., Morgan Stanley and Co.
Incorporated and other brokers and dealers that are affiliates of MSDW Advisors.
It is not anticipated that the portfolio trading will result in the Fund's
portfolio turnover rate exceeding 200%. A high portfolio turnover rate may
increase the Fund's capital gains, which are passed along to Fund shareholders
as distributions. This, in turn, may increase your
 
10
<PAGE>
tax liability as a Fund shareholder. A more extensive discussion of the Fund's
portfolio brokerage policies is set forth in the Statement of Additional
Information. Except as specifically noted, all investment objectives, policies
and practices discussed above are not fundamental policies of the Fund and, as
such, may be changed without shareholder approval.
 
RISK CONSIDERATIONS
 
An increase in prevailing levels of interest rates will generally reduce the
value of securities in the Fund's portfolio, while a decline in rates will
generally increase the value of these securities. As a result, the fluctuations
or changes in interest rates will cause the Fund's net asset value to rise and
fall, in an inverse relationship; however, the income received by the Fund from
its portfolio securities will not be affected. Because yields on debt securities
available for purchase by a Fund vary over time, no specific yield on shares of
the Fund can be assured. In addition, if the bonds in the Fund's portfolio
contain call, prepayment or redemption provisions, during a period of declining
interest rates, these securities are likely to be redeemed, and the Fund will
probably be unable to replace them with securities having an equal yield.
 
YEAR 2000.  The investment management services provided to the Fund by the
Investment Manager and the services provided to shareholders by the Distributor
and the Transfer Agent depend on the smooth functioning of their computer
systems. Many computer software systems in use today cannot recognize the year
2000, but revert to 1900 or some other date, due to the manner in which dates
were encoded and calculated. That failure could have a negative impact on the
handling of securities trades, pricing and account services. The Investment
Manager, the Distributor and the Transfer Agent have been actively working on
necessary changes to their own computer systems to prepare for the year 2000 and
expect that their systems will be adapted before that date, but there can be no
assurance that they will be successful, or that interaction with other
non-complying computer systems will not impair their services at that time.
 
    In addition, it is possible that the markets for securities in which the
Fund invests may be detrimentally affected by computer failures throughout the
financial services industry beginning January 1, 2000. Improperly functioning
trading systems may result in settlement problems and liquidity issues. In
addition, corporate and governmental data processing errors may result in
production problems for individual companies and overall economic uncertainties.
Earnings of individual issuers will be affected by remediation costs, which may
be substantial and may be reported inconsistently in U.S. and foreign financial
statements. Accordingly, the Fund's investments may be adversely affected.
 
    For additional risk disclosure, please refer to the "Investment Objective
and Policies" section of the Prospectus and to the "Investment Practices and
Policies" section in the Statement of Additional Information.
 
INVESTMENT RESTRICTIONS
- --------------------------------------------------------------------------------
 
The investment restrictions listed below are among the restrictions which have
been adopted by the Fund as fundamental policies. Under the Investment Company
Act of 1940, as amended (the "Act"), a fundamental policy may not be changed
without the vote of a majority of the outstanding voting securities of the Fund,
as defined in the Act. For purposes of the following limitations: (i) all
percentage limitations apply immediately after a purchase or initial investment,
and (ii) any subsequent change in any applicable percentage resulting from
market fluctuations or other changes in total or net assets does not require
elimination of any security from the portfolio.
 
    The Fund may not:
 
        1. Invest more than 5% of the value of its total assets in the
    securities of any one issuer (other than obligations issued or guaranteed by
    the United States Government, its agencies or instrumentalities).
 
        2. Purchase more than 10% of all outstanding voting securities or any
    class of securities of any one issuer.
 
        3. Invest 25% or more of the value of its total assets in securities of
    issuers in any one industry. This restriction does not apply to obligations
    issued or guaranteed by the United States Government or its agencies or
    instrumentalities.
 
        4. Invest more than 10% of its total assets in "illiquid securities"
    (securities for which market quotations are not readily available) and
    repurchase agreements which have a maturity of longer than seven days.
 
        5. Invest more than 5% of the value of its total assets in securities of
    issuers having a record, together with predecessors, of less than three
    years of continuous operation. This restriction shall not apply to any
    obligation issued or guaranteed by the United States Government, its
    agencies or instrumentalities.
 
        6. Borrow money, except that the Fund may borrow from banks for
    temporary or emergency purposes in an amount up to 5% (taken at the lower of
    cost or current value) of its total assets (not including the
 
                                                                              11
<PAGE>
    amount borrowed), and may enter into reverse repurchase agreements in an
    amount not exceeding 10% of the Fund's total assets.
 
    Notwithstanding any other investment policy or restriction, the Fund may
seek to achieve its investment objective by investing all or substantially all
of its assets in another investment company having substantially the same
investment objective and policies as the Fund.
 
PURCHASE OF FUND SHARES
- --------------------------------------------------------------------------------
 
GENERAL
 
The Fund offers each class of its shares for sale to the public on a continuous
basis. Pursuant to a Distribution Agreement between the Fund and Morgan Stanley
Dean Witter Distributors Inc. ("MSDW Distributors" or the "Distributor"), an
affiliate of the Investment Manager, shares of the Fund are distributed by the
Distributor and offered by Dean Witter Reynolds Inc. ("DWR"), a selected dealer
and a subsidiary of Morgan Stanley Dean Witter & Co., and other dealers who have
entered into selected dealer agreements with the Distributor ("Selected
Broker-Dealers"). It is anticipated that DWR will undergo a change of corporate
name which is expected to incorporate the brand name of "Morgan Stanley Dean
Witter," pending approval of various regulatory authorities. The principal
executive office of the Distributor is located at Two World Trade Center, New
York, New York 10048.
 
    The Fund offers four classes of shares (each, a "Class"). Class A shares are
sold to investors with an initial sales charge that declines to zero for larger
purchases; however, Class A shares sold without an initial sales charge are
subject to a contingent deferred sales charge ("CDSC") of 1.0% if redeemed
within one year of purchase, except for certain specific circumstances. Class B
shares are sold without an initial sales charge but are subject to a CDSC
(scaled down from 5.0% to 1.0%) payable upon most redemptions within six years
after purchase. (Class B shares purchased by certain qualified plans are subject
to a CDSC scaled down from 2.0% to 1.0% if redeemed within three years after
purchase.) Class C shares are sold without an initial sales charge but are
subject to a CDSC of 1.0% on most redemptions made within one year after
purchase. Class D shares are sold without an initial sales charge or CDSC and
are available only to investors meeting an initial investment minimum of $5
million ($25 million for certain qualified plans), and to certain other limited
categories of investors. At the discretion of the Board of Trustees of the Fund,
Class A shares may be sold to categories of investors in addition to those set
forth in this prospectus at net asset value without a front-end sales charge,
and Class D shares may be sold to certain other categories of investors, in each
case as may be described in the then current prospectus of the Fund. See
"Alternative Purchase Arrangements-- Selecting a Particular Class" for a
discussion of factors to consider in selecting which Class of shares to
purchase.
 
    The minimum initial purchase is $1,000 for each Class of shares, although
Class D shares are only available to persons investing $5 million ($25 million
for certain qualified plans), or more and to certain other limited categories of
investors. For the purpose of meeting the minimum $5 million (or $25 million)
initial investment for Class D shares, and subject to the $1,000 minimum initial
investment for each Class of the Fund, an investor's existing holdings of Class
A shares of the Fund and other Morgan Stanley Dean Witter Funds that are
multiple class funds ("Morgan Stanley Dean Witter Multi-Class Funds") and shares
of Morgan Stanley Dean Witter Funds sold with a front-end sales charge ("FSC
Funds") and concurrent investments in Class D shares of the Fund and other
Morgan Stanley Dean Witter Multi-Class Funds will be aggregated. Subsequent
purchases of $100 or more may be made by sending a check, payable to Morgan
Stanley Dean Witter Intermediate Income Securities, directly to Morgan Stanley
Dean Witter Trust FSB (the "Transfer Agent" or "MSDW Trust") at P.O. Box 1040,
Jersey City, NJ 07303 or by contacting a Morgan Stanley Dean Witter Financial
Advisor or other Selected Broker-Dealer representative. When purchasing shares
of the Fund, investors must specify whether the purchase is for Class A, Class
B, Class C or Class D shares. If no Class is specified, the Transfer Agent will
not process the transaction until the proper Class is identified. The minimum
initial purchase in the case of investments through EasyInvest-SM-, an automatic
purchase plan (see "Shareholder Services"), is $100, provided that the schedule
of automatic investments will result in investments totalling at least $1,000
within the first twelve months. The minimum initial purchase in the case of an
"Education IRA" is $500, if the Distributor has reason to believe that
additional investments will increase the investment in the account to $1,000
within three years. In the case of investments pursuant to (i) Systematic
Payroll Deduction Plans (including Individual Retirement Plans), (ii) the MSDW
Advisors mutual fund asset allocation program and (iii) fee-based programs
approved by the Distributor, pursuant to which participants pay an asset based
fee for services in the nature of investment advisory, administrative and/or
brokerage services, the Fund, in its discretion, may accept investments without
regard to any minimum amounts which would otherwise be required, provided, in
the case of Systematic Payroll Deduction Plans, that the Distributor has reason
to believe that additional investments will increase the investment in all
accounts under such Plans to at least $1,000. Certificates
 
12
<PAGE>
for shares purchased will not be issued unless a request is made by the
shareholder in writing to the Transfer Agent.
 
    Shares of the Fund are sold through the Distributor on a normal three
business day settlement basis; that is, payment is due on the third business day
(settlement date) after the order is placed with the Distributor. Shares of the
Fund purchased through the Distributor are entitled to dividends beginning on
the next business day following settlement date. Since DWR and other Selected
Broker-Dealers forward investors' funds on settlement date, they will benefit
from the temporary use of the funds where payment is made prior thereto. Shares
purchased through the Transfer Agent are entitled to dividends beginning on the
next business day following receipt of an order. As noted above, orders placed
directly with the Transfer Agent must be accompanied by payment. Investors will
be entitled to receive capital gains distributions if their order is received by
the close of business on the day prior to the record date for such
distributions. Sales personnel of a Selected Broker-Dealer are compensated for
selling shares of the Fund at the time of their sale by the Distributor or any
of its affiliates and/or the Selected Broker-Dealer. In addition, some sales
personnel of the Selected Broker-Dealer will receive various types of non-cash
compensation as special sales incentives, including trips, educational and/or
business seminars and merchandise. The Fund and the Distributor reserve the
right to reject any purchase orders.
 
ALTERNATIVE PURCHASE ARRANGEMENTS
 
The Fund offers several Classes of shares to investors designed to provide them
with the flexibility of selecting an investment best suited to their needs. The
general public is offered three Classes of shares: Class A shares, Class B
shares and Class C shares, which differ principally in terms of sales charges
and rate of expenses to which they are subject. A fourth Class of shares, Class
D shares, is offered only to limited categories of investors (see "No Load
Alternative--Class D Shares" below).
 
    Each Class A, Class B, Class C or Class D share of the Fund represents an
identical interest in the investment portfolio of the Fund except that Class A,
Class B and Class C shares bear the expenses of the ongoing shareholder service
fees, Class B and Class C shares bear the expenses of the ongoing distribution
fees and Class A, Class B and Class C shares which are redeemed subject to a
CDSC bear the expense of the additional incremental distribution costs resulting
from the CDSC applicable to shares of those Classes. The ongoing distribution
fees that are imposed on Class A, Class B and Class C shares will be imposed
directly against those Classes and not against all assets of the Fund and,
accordingly, such charges against one Class will not affect the net asset value
of any other Class or have any impact on investors choosing another sales charge
option. See "Plan of Distribution" and "Redemptions and Repurchases."
 
    Set forth below is a summary of the differences between the Classes and the
factors an investor should consider when selecting a particular Class. This
summary is qualified in its entirety by detailed discussion of each Class that
follows this summary.
 
CLASS A SHARES.  Class A shares are sold at net asset value plus an initial
sales charge of up to 4.25%. The initial sales charge is reduced for certain
purchases. Investments of $1 million or more (and investments by certain other
limited categories of investors) are not subject to any sales charges at the
time of purchase but are subject to a CDSC of 1.0% on redemptions made within
one year after purchase, except for certain specific circumstances. Class A
shares are also subject to a 12b-1 fee of up to 0.25% of the average daily net
assets of the Class. See "Initial Sales Charge Alternative--Class A Shares."
 
CLASS B SHARES.  Class B shares are offered at net asset value with no initial
sales charge but are subject to a CDSC (scaled down from 5.0% to 1.0%) if
redeemed within six years of purchase. (Class B shares purchased by certain
qualified plans are subject to a CDSC scaled down from 2.0% to 1.0% if redeemed
within three years after purchase.) This CDSC may be waived for certain
redemptions. Class B shares are also subject to an annual 12b-1 fee of 0.85% of
the lesser of: (a) the average daily aggregate gross sales of the Fund's Class B
shares since the inception of the Fund (not including reinvestments of dividends
or capital gains distributions), less the average daily aggregate net asset
value of the Fund's Class B shares redeemed since the Fund's inception upon
which a CDSC has been imposed or waived, or (b) the average daily net assets of
Class B. The Class B shares' distribution fee will cause that Class to have
higher expenses and pay lower dividends than Class A or Class D shares.
 
    After approximately ten (10) years, Class B shares will convert
automatically to Class A shares of the Fund, based on the relative net asset
values of the shares of the two Classes on the conversion date. In addition, a
certain portion of Class B shares that have been acquired through the
reinvestment of dividends and distributions will be converted at that time. See
"Contingent Deferred Sales Charge Alternative--Class B Shares."
 
CLASS C SHARES.  Class C shares are sold at net asset value with no initial
sales charge but are subject to a CDSC of 1.0% on redemptions made within one
year after purchase. This CDSC may be waived for certain redemptions. They are
subject to an annual 12b-1 fee of up to 0.85% of the average daily net assets of
the Class C shares. The Class C shares' distribution fee may cause that Class to
have higher expenses and pay lower dividends than Class A or Class D shares. See
"Level Load Alternative--Class C Shares."
 
CLASS D SHARES.  Class D shares are available only to limited categories of
investors (see "No Load Alternative-- Class D Shares" below). Class D shares are
sold at net asset value with no initial sales charge or CDSC. They are
 
                                                                              13
<PAGE>
not subject to any 12b-1 fees. See "No Load Alternative-- Class D Shares."
 
SELECTING A PARTICULAR CLASS.  In deciding which Class of Fund shares to
purchase, investors should consider the following factors, as well as any other
relevant facts and circumstances:
 
    The decision as to which Class of shares is more beneficial to an investor
depends on the amount and intended length of his or her investment. Investors
who prefer an initial sales charge alternative may elect to purchase Class A
shares. Investors qualifying for significantly reduced or, in the case of
purchases of $1 million or more, no initial sales charges may find Class A
shares particularly attractive because similar sales charge reductions are not
available with respect to Class B or Class C shares. Moreover, Class A shares
are subject to lower ongoing expenses than are Class B or Class C shares over
the term of the investment. As an alternative, Class B and Class C shares are
sold without any initial sales charge so the entire purchase price is
immediately invested in the Fund. Any investment return on these additional
investment amounts may partially or wholly offset the higher annual expenses of
these Classes. Because the Fund's future return cannot be predicted, however,
there can be no assurance that this would be the case.
 
    Finally, investors should consider the effect of the CDSC period and any
conversion rights of the Classes in the context of their own investment time
frame. For example, although Class C shares are subject to a significantly lower
CDSC upon redemptions, they do not, unlike Class B shares, convert into Class A
shares after approximately ten years, and, therefore, are subject to an ongoing
12b-1 fee of 0.85% (rather than the 0.25% fee applicable to Class A shares) for
an indefinite period of time. Thus, Class B shares may be more attractive than
Class C shares to investors with longer term investment outlooks. Other
investors, however, may elect to purchase Class C shares if, for example, they
determine that they do not wish to be subject to a front-end sales charge and
they are uncertain as to the length of time they intend to hold their shares.
 
    For the purpose of meeting the $5 million (or $25 million) minimum
investment amount for Class D shares, holdings of Class A shares in all Morgan
Stanley Dean Witter Multi-Class Funds, shares of FSC Funds and shares of Morgan
Stanley Dean Witter Funds for which such shares have been exchanged will be
included together with the current investment amount.
 
    Sales personnel may receive different compensation for selling each Class of
shares. Investors should understand that the purpose of a CDSC is the same as
that of the initial sales charge in that the sales charges applicable to each
Class provide for the financing of the distribution of shares of that Class.
 
    Set forth below is a chart comparing the sales charge, 12b-1 fees and
conversion options applicable to each Class of shares:
 
<TABLE>
<CAPTION>
                                                   CONVERSION
  CLASS          SALES CHARGE         12b-1 FEE      FEATURE
<C>        <S>                        <C>        <C>
    A      Maximum 4.25% initial        0.25%          No
           sales charge reduced for
           purchases of $25,000 and
           over; shares sold without
           an initial sales charge
           generally subject to a
           1.0% CDSC during first
           year.
    B      Maximum 5.0% CDSC during     0.85%    B shares
           the first year decreasing             convert to A
           to 0 after six years                  shares
                                                 automatically
                                                 after
                                                 approximately
                                                 ten years
    C      1.0% CDSC during first       0.85%          No
           year
    D                None               None           No
</TABLE>
 
    See "Purchase of Fund Shares" and "The Fund and its Management" for a
complete description of the sales charges and service and distribution fees for
each Class of shares and "Determination of Net Asset Value," "Dividends,
Distributions and Taxes" and "Shareholder Services--Exchange Privilege" for
other differences between the Classes of shares.
 
INITIAL SALES CHARGE ALTERNATIVE-- CLASS A SHARES
 
Class A shares are sold at net asset value plus an initial sales charge. In some
cases, reduced sales charges may be available, as described below. Investments
of $1 million or more (and investments by certain other limited categories of
investors) are not subject to any sales charges at the time of purchase but are
subject to a CDSC of 1.0% on redemptions made within one year after purchase
(calculated from the last day of the month in which the shares were purchased),
except for certain specific circumstances. The CDSC will be assessed on an
amount equal to the lesser of the current market value or the cost of the shares
being redeemed. The CDSC will not be imposed (i) in the circumstances set forth
below in the section "Contingent Deferred Sales Charge Alternative--Class B
Shares-- CDSC Waivers," except that the references to six years in the first
paragraph of that section shall mean one year in the case of Class A shares, and
(ii) in the circumstances identified in the section "Additional Net Asset Value
Purchase Options" below. Class A shares are also subject to an annual 12b-1 fee
of up to 0.25% of the average daily net assets of the Class.
 
    The offering price of Class A shares will be the net asset value per share
next determined following receipt of an order (see "Determination of Net Asset
Value" below),
 
14
<PAGE>
plus a sales charge (expressed as a percentage of the offering price) on a
single transaction as shown in the following table:
 
<TABLE>
<CAPTION>
                                         SALES CHARGE
                             -------------------------------------
                               PERCENTAGE OF       APPROXIMATE
         AMOUNT OF            PUBLIC OFFERING     PERCENTAGE OF
    SINGLE TRANSACTION             PRICE         AMOUNT INVESTED
- ---------------------------  -----------------  ------------------
<S>                          <C>                <C>
Less than $25,000..........          4.25%               4.44%
$25,000 but less
  than $50,000.............          4.00%               4.17%
$50,000 but less
  than $100,000............          3.50%               3.63%
$100,000 but less
  than $250,000............          2.75%               2.83%
$250,000 but less
  than $1 million..........          1.75%               1.78%
$1 million and over........             0                   0
</TABLE>
 
    Upon notice to all Selected Broker-Dealers, the Distributor may reallow up
to the full applicable sales charge as shown in the above schedule during
periods specified in such notice. During periods when 90% or more of the sales
charge is reallowed, such Selected Broker-Dealers may be deemed to be
underwriters as that term is defined in the Securities Act of 1933.
 
    The above schedule of sales charges is applicable to purchases in a single
transaction by, among others: (a) an individual; (b) an individual, his or her
spouse and their children under the age of 21 purchasing shares for his, her or
their own accounts; (c) a trustee or other fiduciary purchasing shares for a
single trust estate or a single fiduciary account; (d) a pension, profit-sharing
or other employee benefit plan qualified or non-qualified under Section 401 of
the Internal Revenue Code; (e) tax-exempt organizations enumerated in Section
501(c)(3) or (13) of the Internal Revenue Code; (f) employee benefit plans
qualified under Section 401 of the Internal Revenue Code of a single employer or
of employers who are "affiliated persons" of each other within the meaning of
Section 2(a)(3)(c) of the Act; and for investments in Individual Retirement
Accounts of employees of a single employer through Systematic Payroll Deduction
plans; or (g) any other organized group of persons, whether incorporated or not,
provided the organization has been in existence for at least six months and has
some purpose other than the purchase of redeemable securities of a registered
investment company at a discount.
 
COMBINED PURCHASE PRIVILEGE.  Investors may have the benefit of reduced sales
charges in accordance with the above schedule by combining purchases of Class A
shares of the Fund in single transactions with the purchase of Class A shares of
other Morgan Stanley Dean Witter Multi-Class Funds and shares of FSC Funds. The
sales charge payable on the purchase of the Class A shares of the Fund, the
Class A shares of the other Morgan Stanley Dean Witter Multi-Class Funds and the
shares of the FSC Funds will be at their respective rates applicable to the
total amount of the combined concurrent purchases of such shares.
 
RIGHT OF ACCUMULATION.  The above persons and entities may benefit from a
reduction of the sales charges in accordance with the above schedule if the
cumulative net asset value of Class A shares purchased in a single transaction,
together with shares of the Fund and other Morgan Stanley Dean Witter Funds
previously purchased at a price including a front-end sales charge (including
shares of the Fund and other Morgan Stanley Dean Witter Funds acquired in
exchange for those shares, and including in each case shares acquired through
reinvestment of dividends and distributions), which are held at the time of such
transaction, amounts to $25,000 or more. If such investor has a cumulative net
asset value of shares of FSC Funds and Class A and Class D shares that, together
with the current investment amount, is equal to at least $5 million ($25 million
for certain qualified plans), such investor is eligible to purchase Class D
shares subject to the $1,000 minimum initial investment requirement of that
Class of the Fund. See "No Load Alternative--Class D Shares" below.
 
    The Distributor must be notified by DWR or a Selected Broker-Dealer or the
shareholder at the time a purchase order is placed that the purchase qualifies
for the reduced charge under the Right of Accumulation. Similar notification
must be made in writing by the dealer or shareholder when such an order is
placed by mail. The reduced sales charge will not be granted if: (a) such
notification is not furnished at the time of the order; or (b) a review of the
records of the Selected Broker-Dealer or the Transfer Agent fails to confirm the
investor's represented holdings.
 
LETTER OF INTENT.  The foregoing schedule of reduced sales charges will also be
available to investors who enter into a written Letter of Intent providing for
the purchase, within a thirteen-month period, of Class A shares of the Fund from
DWR or other Selected Broker-Dealers. The cost of Class A shares of the Fund or
shares of other Morgan Stanley Dean Witter Funds which were previously purchased
at a price including a front-end sales charge during the 90-day period prior to
the date of receipt by the Distributor of the Letter of Intent, or of Class A
shares of the Fund or shares of other Morgan Stanley Dean Witter Funds acquired
in exchange for shares of such funds purchased during such period at a price
including a front-end sales charge, which are still owned by the shareholder,
may also be included in determining the applicable reduction.
 
ADDITIONAL NET ASSET VALUE PURCHASE OPTIONS.  In addition to investments of $1
million or more, Class A shares also may be purchased at net asset value by the
following:
 
    (1) trusts for which MSDW Trust (which is an affiliate of the Investment
Manager) provides discretionary trustee services;
 
    (2) persons participating in a fee-based program approved by the
Distributor, pursuant to which such persons pay an asset based fee for services
in the nature of investment advisory, administrative and/or brokerage services
(such investments are subject to all of the terms and
 
                                                                              15
<PAGE>
conditions of such programs, which may include termination fees, mandatory
redemption upon termination and such other circumstances as specified in the
programs' agreements, and restrictions on transferability of Fund shares);
 
    (3) employer-sponsored 401(k) and other plans qualified under Section 401(a)
of the Internal Revenue Code ("Qualified Retirement Plans") with at least 200
eligible employees and for which MSDW Trust serves as Trustee or DWR's
Retirement Plan Services serves as recordkeeper pursuant to a written
Recordkeeping Services Agreement;
 
    (4) Qualified Retirement Plans for which MSDW Trust serves as Trustee or
DWR's Retirement Plan Services serves as recordkeeper pursuant to a written
Recordkeeping Services Agreement whose Class B shares have converted to Class A
shares, regardless of the plan's asset size or number of eligible employees;
 
    (5) investors who are clients of a Morgan Stanley Dean Witter Financial
Advisor who joined Morgan Stanley Dean Witter from another investment firm
within six months prior to the date of purchase of Fund shares by such
investors, if the shares are being purchased with the proceeds from a redemption
of shares of an open-end proprietary mutual fund of the Financial Advisor's
previous firm which imposed either a front-end or deferred sales charge,
provided such purchase was made within sixty days after the redemption and the
proceeds of the redemption had been maintained in the interim in cash or a money
market fund; and
 
    (6) other categories of investors, at the discretion of the Board, as
disclosed in the then current prospectus of the Fund.
 
    No CDSC will be imposed on redemptions of shares purchased pursuant to
paragraphs (1), (2) or (5), above.
 
    For further information concerning purchases of the Fund's shares, contact
DWR or another Selected Broker-Dealer or consult the Statement of Additional
Information.
 
CONTINGENT DEFERRED SALES CHARGE ALTERNATIVE--CLASS B SHARES
 
Class B shares are sold at net asset value next determined without an initial
sales charge so that the full amount of an investor's purchase payment may be
immediately invested in the Fund. A CDSC, however, will be imposed on most Class
B shares redeemed within six years after purchase. The CDSC will be imposed on
any redemption of shares if after such redemption the aggregate current value of
a Class B account with the Fund falls below the aggregate amount of the
investor's purchase payments for Class B shares made during the six years (or,
in the case of shares held by certain Qualified Retirement Plans, three years)
preceding the redemption. In addition, Class B shares are subject to an annual
12b-1 fee of 0.85% of the lesser of: (a) the average daily aggregate gross sales
of the Fund's Class B shares since the inception of the Fund (not including
reinvestments of dividends or capital gains distributions), less the average
daily aggregate net asset value of the Fund's Class B shares redeemed since the
Fund's inception upon which a CDSC has been imposed or waived, or (b) the
average daily net assets of Class B.
 
    Except as noted below, Class B shares of the Fund which are held for six
years or more after purchase (calculated from the last day of the month in which
the shares were purchased) will not be subject to any CDSC upon redemption.
Shares redeemed earlier than six years after purchase may, however, be subject
to a CDSC which will be a percentage of the dollar amount of shares redeemed and
will be assessed on an amount equal to the lesser of the current market value or
the cost of the shares being redeemed. The size of this percentage will depend
upon how long the shares have been held, as set forth in the following table:
 
<TABLE>
<CAPTION>
          YEAR SINCE PURCHASE             CDSC AS A PERCENTAGE
              PAYMENT MADE                 OF AMOUNT REDEEMED
- ----------------------------------------  ---------------------
<S>                                       <C>
First...................................             5.0%
Second..................................             4.0%
Third...................................             3.0%
Fourth..................................             2.0%
Fifth...................................             2.0%
Sixth...................................             1.0%
Seventh and thereafter..................             None
</TABLE>
 
    In the case of Class B shares of the Fund purchased on or after July 28,
1997 by Qualified Retirement Plans for which MSDW Trust serves as Trustee or
DWR's Retirement Plan Services serves as recordkeeper pursuant to a written
Recordkeeping Services Agreement, shares held for three years or more after
purchase (calculated as described in the paragraph above) will not be subject to
any CDSC upon redemption. However, shares redeemed earlier than three years
after purchase may be subject to a CDSC (calculated as described in the
paragraph above), the percentage of which will depend on how long the shares
have been held, as set forth in the following table:
 
<TABLE>
<CAPTION>
          YEAR SINCE PURCHASE             CDSC AS A PERCENTAGE
              PAYMENT MADE                 OF AMOUNT REDEEMED
- ----------------------------------------  ---------------------
<S>                                       <C>
First...................................             2.0%
Second..................................             2.0%
Third...................................             1.0%
Fourth and thereafter...................             None
</TABLE>
 
CDSC WAIVERS.  A CDSC will not be imposed on: (i) any amount which represents an
increase in value of shares purchased within the six years (or, in the case of
shares held by certain Qualified Retirement Plans, three years) preceding the
redemption; (ii) the current net asset value of shares purchased more than six
years (or, in the case of shares held by certain Qualified Retirement Plans,
three years) prior to the redemption; and (iii) the current net asset value of
shares purchased through reinvestment of dividends or distributions and/or
shares acquired in
 
16
<PAGE>
exchange for shares of FSC Funds or of other Morgan Stanley Dean Witter Funds
acquired in exchange for such shares. Moreover, in determining whether a CDSC is
applicable it will be assumed that amounts described in (i), (ii) and (iii)
above (in that order) are redeemed first.
 
    In addition, the CDSC, if otherwise applicable, will be waived in the case
of:
 
    (1) redemptions of shares held at the time a shareholder dies or becomes
disabled, only if the shares are:  (a) registered either in the name of an
individual shareholder (not a trust), or in the names of such shareholder and
his or her spouse as joint tenants with right of survivorship; or  (b) held in a
qualified corporate or self-employed retirement plan, Individual Retirement
Account ("IRA") or Custodial Account under Section 403(b)(7) of the Internal
Revenue Code ("403(b) Custodial Account"), provided in either case that the
redemption is requested within one year of the death or initial determination of
disability;
 
    (2) redemptions in connection with the following retirement plan
distributions:  (a) lump-sum or other distributions from a qualified corporate
or self-employed retirement plan following retirement (or, in the case of a "key
employee" of a "top heavy" plan, following attainment of age 59 1/2); (b)
distributions from an IRA or 403(b) Custodial Account following attainment of
age 59 1/2; or  (c) a tax-free return of an excess contribution to an IRA;
 
    (3) all redemptions of shares held for the benefit of a participant in a
Qualified Retirement Plan which offers investment companies managed by the
Investment Manager or its subsidiary, MSDW Services, as self-directed investment
alternatives and for which MSDW Trust serves as Trustee or DWR's Retirement Plan
Services serves as recordkeeper pursuant to a written Recordkeeping Services
Agreement ("Eligible Plan"), provided that either:  (a) the plan continues to be
an Eligible Plan after the redemption; or  (b) the redemption is in connection
with the complete termination of the plan involving the distribution of all plan
assets to participants; and
 
    (4) certain redemptions pursuant to the Fund's Systematic Withdrawal Plan
(see "Shareholder Services--Systematic Withdrawal Plan").
 
    With reference to (1) above, for the purpose of determining disability, the
Distributor utilizes the definition of disability contained in Section 72(m)(7)
of the Internal Revenue Code, which relates to the inability to engage in
gainful employment. With reference to (2) above, the term "distribution" does
not encompass a direct transfer of IRA, 403(b) Custodial Account or retirement
plan assets to a successor custodian or trustee. All waivers will be granted
only following receipt by the Distributor of confirmation of the shareholder's
entitlement.
 
CONVERSION TO CLASS A SHARES.  All shares of the Fund held prior to July 28,
1997 (other than shares held by certain employee benefit plans established by
DWR and its affiliate, SPS Transaction Services, Inc.) have been designated
Class B shares. Shares held before May 1, 1997 that have been designated Class B
shares will convert to Class A shares in May, 2007. In all other instances Class
B shares will convert automatically to Class A shares, based on the relative net
asset values of the shares of the two Classes on the conversion date, which will
be approximately ten (10) years after the date of the original purchase. The ten
year period is calculated from the last day of the month in which the shares
were purchased or, in the case of Class B shares acquired through an exchange or
a series of exchanges, from the last day of the month in which the original
Class B shares were purchased, provided that shares originally purchased before
May 1, 1997 will convert to Class A shares in May, 2007. The conversion of
shares purchased on or after May 1, 1997 will take place in the month following
the tenth anniversary of the purchase. There will also be converted at that time
such proportion of Class B shares acquired through automatic reinvestment of
dividends and distributions owned by the shareholder as the total number of his
or her Class B shares converting at the time bears to the total number of
outstanding Class B shares purchased and owned by the shareholder. In the case
of Class B shares held by a Qualified Retirement Plan for which MSDW Trust
serves as Trustee or DWR's Retirement Plan Services serves as recordkeeper
pursuant to a written Recordkeeping Services Agreement, the plan is treated as a
single investor and all Class B shares will convert to Class A shares on the
conversion date of the first shares of a Morgan Stanley Dean Witter Multi-Class
Fund purchased by that plan. In the case of Class B shares previously exchanged
for shares of an "Exchange Fund" (see "Shareholder Services--Exchange
Privilege"), the period of time the shares were held in the Exchange Fund
(calculated from the last day of the month in which the Exchange Fund shares
were acquired) is excluded from the holding period for conversion. If those
shares are subsequently re-exchanged for Class B shares of a Morgan Stanley Dean
Witter Multi-Class Fund, the holding period resumes on the last day of the month
in which Class B shares are reacquired.
 
    If a shareholder has received share certificates for Class B shares, such
certificates must be delivered to the Transfer Agent at least one week prior to
the date for conversion. Class B shares evidenced by share certificates that are
not received by the Transfer Agent at least one week prior to any conversion
date will be converted into Class A shares on the next scheduled conversion date
after such certificates are received.
 
    Effectiveness of the conversion feature is subject to the continuing
availability of a ruling of the Internal Revenue Service or an opinion of
counsel that (i) the conversion of shares does not constitute a taxable event
under the Internal Revenue Code, (ii) Class A shares received on conversion will
have a basis equal to the shareholder's basis in the converted Class B shares
immediately prior to the
 
                                                                              17
<PAGE>
conversion, and (iii) Class A shares received on conversion will have a holding
period that includes the holding period of the converted Class B shares. The
conversion feature may be suspended if the ruling or opinion is no longer
available. In such event, Class B shares would continue to be subject to Class B
12b-1 fees.
 
LEVEL LOAD ALTERNATIVE-- CLASS C SHARES
 
Class C shares are sold at net asset value next determined without an initial
sales charge but are subject to a CDSC of 1.0% on most redemptions made within
one year after purchase (calculated from the last day of the month in which the
shares were purchased). The CDSC will be assessed on an amount equal to the
lesser of the current market value or the cost of the shares being redeemed. The
CDSC will not be imposed in the circumstances set forth above in the section
"Contingent Deferred Sales Charge Alternative-- Class B Shares--CDSC Waivers,"
except that the references to six years in the first paragraph of that section
shall mean one year in the case of Class C shares. Class C shares are subject to
an annual 12b-1 fee of up to 0.85% of the average daily net assets of the Class.
Unlike Class B shares, Class C shares have no conversion feature and,
accordingly, an investor that purchases Class C shares will be subject to 12b-1
fees applicable to Class C shares for an indefinite period subject to annual
approval by the Fund's Board of Trustees and regulatory limitations.
 
NO LOAD ALTERNATIVE-- CLASS D SHARES
 
Class D shares are offered without any sales charge on purchase or redemption
and without any 12b-1 fee. Class D shares are offered only to investors meeting
an initial investment minimum of $5 million ($25 million for Qualified
Retirement Plans for which MSDW Trust serves as Trustee or DWR's Retirement Plan
Services serves as recordkeeper pursuant to a written Recordkeeping Services
Agreement) and the following categories of investors: (i) investors
participating in the MSDW Advisors mutual fund asset allocation program pursuant
to which such persons pay an asset based fee; (ii) persons participating in a
fee-based program approved by the Distributor, pursuant to which such persons
pay an asset based fee for services in the nature of investment advisory,
administrative and/or brokerage services (subject to all of the terms and
conditions of such programs, referred to in (i) and (ii) above, which may
include termination fees, mandatory redemption upon termination and such other
circumstances as specified in the programs agreements, and restrictions on
transferability of Fund shares); (iii) employee benefit plans maintained by
Morgan Stanley Dean Witter & Co. or any of its subsidiaries for the benefit of
certain employees of Morgan Stanley Dean Witter & Co. and its subsidiaries; (iv)
certain Unit Investment Trusts sponsored by DWR; (v) certain other open-end
investment companies whose shares are distributed by the Distributor; (vi)
investors who were shareholders of Dean Witter Retirement Series on September
11, 1998 (with respect to additional purchases for their former Dean Witter
Retirement Series accounts); and (vii) other categories of investors, at the
discretion of the Board, as disclosed in the then current prospectus of the
Fund. Investors who require a $5 million (or $25 million) minimum initial
investment to qualify to purchase Class D shares may satisfy that requirement by
investing that amount in a single transaction in Class D shares of the Fund and
other Morgan Stanley Dean Witter Multi-Class Funds, subject to the $1,000
minimum initial investment required for that Class of the Fund. In addition, for
the purpose of meeting the $5 million (or $25 million) minimum investment
amount, holdings of Class A shares in all Morgan Stanley Dean Witter Multi-Class
Funds, shares of FSC Funds and shares of Morgan Stanley Dean Witter Funds for
which such shares have been exchanged will be included together with the current
investment amount. If a shareholder redeems Class A shares and purchases Class D
shares, such redemption may be a taxable event.
 
PLAN OF DISTRIBUTION
 
The Fund has adopted a Plan of Distribution pursuant to Rule 12b-1 under the Act
with respect to the distribution of Class A, Class B and Class C shares of the
Fund. In the case of Class A and Class C shares, the Plan provides that the Fund
will reimburse the Distributor and others for the expenses of certain activities
and services incurred by them specifically on behalf of those shares.
Reimbursements for these expenses will be made in monthly payments by the Fund
to the Distributor, which will in no event exceed amounts equal to payments at
the annual rates of 0.25% and 0.85% of the average daily net assets of Class A
and Class C, respectively. In the case of Class B shares, the Plan provides that
the Fund will pay the Distributor a fee, which is accrued daily and paid
monthly, at the annual rate of 0.85% of the lesser of: (a) the average daily
aggregate gross sales of the Fund's Class B shares since the inception of the
Fund (not including reinvestments of dividends or capital gains distributions),
less the average daily aggregate net asset value of the Fund's Class B shares
redeemed since the Fund's inception upon which a CDSC has been imposed or
waived, or (b) the average daily net assets of Class B. The fee is treated by
the Fund as an expense in the year it is accrued. In the case of Class A shares,
the entire amount of the fee currently represents a service fee within the
meaning of the NASD guidelines. In the case of Class B and Class C shares, a
portion of the fee payable pursuant to the Plan, equal to 0.20% and 0.25% of the
average daily net assets of each of these Classes, respectively, is currently
characterized as a service fee. A service fee is a payment made for personal
service and/or the maintenance of shareholder accounts.
 
    Additional amounts paid under the Plan in the case of Class B and Class C
shares are paid to the Distributor for
 
18
<PAGE>
services provided and the expenses borne by the Distributor and others in the
distribution of the shares of those Classes, including the payment of
commissions for sales of the shares of those Classes and incentive compensation
to and expenses of Morgan Stanley Dean Witter Financial Advisors and others who
engage in or support distribution of shares or who service shareholder accounts,
including overhead and telephone expenses; printing and distribution of
prospectuses and reports used in connection with the offering of the Fund's
shares to other than current shareholders; and preparation, printing and
distribution of sales literature and advertising materials. In addition, the
Distributor may utilize fees paid pursuant to the Plan in the case of Class B
shares to compensate DWR and other Selected Broker-Dealers for their opportunity
costs in advancing such amounts, which compensation would be in the form of a
carrying charge on any unreimbursed expenses.
 
    For the fiscal year ended August 31, 1998, Class B shares of the Fund
accrued payments under the Plan amounting to $1,300,714, which amount is equal
to 0.85% of the average daily net assets of Class B for the fiscal year. These
payments were calculated pursuant to clause (b) of the compensation formula
under the Plan. For the fiscal year ended August 31, 1998, Class A and Class C
shares of the Fund accrued payments under the Plan amounting to $4,202 and
$1,925, respectively, which amounts are equal to 0.24% and 0.85% of the average
daily net assets of Class A and Class C, respectively, for the fiscal year.
 
    In the case of Class B shares, at any given time, the expenses in
distributing Class B shares of the Fund may be in excess of the total of (i) the
payments made by the Fund pursuant to the Plan, and (ii) the proceeds of CDSCs
paid by investors upon the redemption of Class B shares. For example, if $1
million in expenses in distributing Class B shares of the Fund had been incurred
and $750,000 had been received as described in (i) and (ii) above, the excess
expense would amount to $250,000. The Distributor has advised the Fund that such
excess amounts, including the carrying charge described above, totalled
$5,232,570 at August 31, 1998, which was equal to 3.45% of the net assets of
Class B on such date. Because there is no requirement under the Plan that the
Distributor be reimbursed for all distribution expenses or any requirement that
the Plan be continued from year to year, such excess amount does not constitute
a liability of the Fund. Although there is no legal obligation for the Fund to
pay expenses incurred in excess of payments made to the Distributor under the
Plan, and the proceeds of CDSCs paid by investors upon redemption of shares, if
for any reason the Plan is terminated the Trustees will consider at that time
the manner in which to treat such expenses. Any cumulative expenses incurred,
but not yet recovered through distribution fees or CDSCs, may or may not be
recovered through future distribution fees or CDSCs.
 
    In the case of Class A and Class C shares, expenses incurred pursuant to the
Plan in any calendar year in excess of 0.25% or 0.85% of the average daily net
assets of Class A or Class C, respectively, will not be reimbursed by the Fund
through payments in any subsequent year, except that expenses representing a
gross sales commission credited to Morgan Stanley Dean Witter Financial Advisors
and other Selected Broker-Dealer representatives at the time of sale may be
reimbursed in the subsequent calendar year. The Distributor has advised the Fund
that unreimbursed expenses representing a gross sales commission credited to
Morgan Stanley Dean Witter Financial Advisors and other Selected Broker-Dealer
representatives at the time of sale totalled $2,528 in the case of Class C at
December 31, 1997, which was equal to 1.05% of the net assets of Class C on such
date, and that there were no such expenses which may be reimbursed in the
subsequent year in the case of Class A on such date. No interest or other
financing charges will be incurred on any Class A or Class C distribution
expenses incurred by the Distributor under the Plan or on any unreimbursed
expenses due to the Distributor pursuant to the Plan.
 
DETERMINATION OF NET ASSET VALUE
 
The net asset value per share is determined once daily at 4:00 p.m., New York
time (or, on days when the New York Stock Exchange closes prior to 4:00 p.m., at
such earlier time), on each day that the New York Stock Exchange is open by
taking the net assets of the Fund, dividing by the number of shares outstanding
and adjusting to the nearest cent. The assets belonging to the Class A, Class B,
Class C and Class D shares will be invested together in a single portfolio. The
net asset value of each Class, however, will be determined separately by
subtracting each Class's accrued expenses and liabilities. The net asset value
per share will not be determined on Good Friday and on such other federal and
non-federal holidays as are observed by the New York Stock Exchange.
 
    In the calculation of the Fund's net asset value: (1) an equity portfolio
security listed or traded on the New York or American Stock Exchange or other
domestic or foreign exchange is valued at its latest sale price on that exchange
prior to the time assets are valued; if there were no sales that day, the
security is valued at the latest bid price (in cases where a security is traded
on more than one exchange, the security is valued on the exchange designated as
the primary market pursuant to procedures adopted by the Trustees); and (2) all
portfolio securities for which over-the-counter market quotations are readily
available are valued at the latest bid price. When market quotations are not
readily available, including circumstances under which it is determined by the
Investment Manager that sale and bid prices are not reflective of a security's
market value, portfolio securities are valued at their fair
 
                                                                              19
<PAGE>
value as determined in good faith under procedures established by and under the
general supervision of the Board of Trustees.
 
    Certain of the Fund's portfolio securities may be valued by an outside
pricing service approved by the Fund's Trustees. The pricing service may utilize
a matrix system incorporating security quality, maturity and coupon as the
evaluation model parameters, and/or research evaluations by its staff, including
review of broker-dealer market price quotations, in determining what it believes
is the fair valuation of the portfolio securities valued by such pricing
service. (See "Determination of Net Asset Value" in the Statement of Additional
Information.)
 
SHAREHOLDER SERVICES
- --------------------------------------------------------------------------------
 
AUTOMATIC INVESTMENT OF DIVIDENDS AND DISTRIBUTIONS.  All income dividends and
capital gains distributions are automatically paid in full and fractional shares
of the applicable Class of the Fund (or, if specified by the shareholder, in
shares of any other open-end Morgan Stanley Dean Witter Fund), unless the
shareholder requests that they be paid in cash. Shares so acquired are acquired
at net asset value and are not subject to the imposition of a front-end sales
charge or a CDSC (see "Redemptions and Repurchases"). Such dividends and
distributions will be paid, at the net asset value per share (without sales
charge), in shares of the applicable Class of the Fund (or in cash if the
shareholder so requests) on the monthly payment date, which will be no later
than the last business day of the month for which the dividend or distribution
is payable. Processing of dividend checks begins immediately following the
monthly payment date. Shareholders who have requested to receive dividends in
cash will normally receive their monthly dividend check during the first ten
days of the following month.
 
EASYINVEST-SM-.  Shareholders may subscribe to EasyInvest, an automatic purchase
plan which provides for any amount from $100 to $5,000 to be transferred
automatically from a checking or savings account, or following redemption of
shares of a Morgan Stanley Dean Witter money market fund, on a semi-monthly,
monthly or quarterly basis, to the Fund's Transfer Agent for investment in
shares of the Fund (see "Purchases of Fund Shares" and "Redemptions and
Repurchases--Involuntary Redemption").
 
INVESTMENT OF DIVIDENDS OR DISTRIBUTIONS RECEIVED IN CASH.  Any shareholder who
receives a cash payment representing a dividend or capital gains distribution
may invest such dividend or distribution in shares of the applicable Class at
the net asset value per share next determined after receipt by the Transfer
Agent, by returning the check or the proceeds to the Transfer Agent within
thirty days after the payment date. Shares so acquired are acquired at net asset
value and are not subject to the imposition of a front-end sales charge or a
CDSC (see "Redemptions and Repurchases").
 
SYSTEMATIC WITHDRAWAL PLAN.  A systematic withdrawal plan (the "Withdrawal
Plan") is available for shareholders whose shares of Morgan Stanley Dean Witter
Funds have an aggregate value of $10,000 or more. Shares of any Fund from which
redemptions will be made pursuant to the Plan must have a value of $1,000 or
more (referred to as a "SWP Fund"). The required share values are determined on
the date the shareholder establishes the Withdrawal Plan. The Withdrawal Plan
provides for monthly, quarterly, semi-annual or annual payments in any amount
not less than $25, or in any whole percentage of the value of the SWP Funds'
shares, on an annualized basis. Any applicable CDSC will be imposed on shares
redeemed under the Withdrawal Plan (see "Purchase of Fund Shares"), except that
the CDSC, if any, will be waived on redemptions under the Withdrawal Plan of up
to 12% annually of the value of each SWP Fund account, based on the share values
next determined after the shareholder establishes the Withdrawal Plan.
Redemptions for which this CDSC waiver policy applies may be in amounts up to 1%
per month, 3% per quarter, 6% semi-annually or 12% annually. Under this CDSC
waiver policy, amounts withdrawn each period will be paid by first redeeming
shares not subject to a CDSC because the shares were purchased by the
reinvestment of dividends or capital gains distributions, the CDSC period has
elapsed or some other waiver of the CDSC applies. If shares subject to a CDSC
must be redeemed, shares held for the longest period of time will be redeemed
first and continuing with shares held the next longest period of time until
shares held the shortest period of time are redeemed. Any shareholder
participating in the Withdrawal Plan will have sufficient shares redeemed from
his or her account so that the proceeds (net of any applicable CDSC) to the
shareholder will be the designated monthly, quarterly, semi-annual or annual
amount.
 
    A shareholder may suspend or terminate participation in the Withdrawal Plan
at any time. A shareholder who has suspended participation may resume payments
under the Withdrawal Plan, without requiring a new determination of the account
value for the 12% CDSC waiver. The Withdrawal Plan may be terminated or revised
at any time by the Fund.
 
    Prior to adding an additional SWP Fund to an existing Withdrawal Plan, the
required $10,000/$1,000 share values must be met, to be calculated on the date
the shareholder adds the additional SWP Fund. However, the addition of a new SWP
Fund will not change the account value for the 12% CDSC waiver for the SWP Funds
already participating in the Withdrawal Plan.
 
    Withdrawal Plan payments should not be considered dividends, yields or
income. If periodic Withdrawal Plan
 
20
<PAGE>
payments continuously exceed net investment income and net capital gains, the
shareholder's original investment will be correspondingly reduced and ultimately
exhausted. Each withdrawal constitutes a redemption of shares and any gain or
loss realized must be recognized for federal income tax purposes.
 
    Shareholders should contact their Morgan Stanley Dean Witter Financial
Advisor or other Selected Broker-Dealer representative or the Transfer Agent for
further information about any of the above services.
 
TAX-SHELTERED RETIREMENT PLANS.  Retirement plans are available for use by
corporations, the self-employed, eligible Individual Retirement Accounts and
Custodial Accounts under Section 403(b)(7) of the Internal Revenue Code.
Adoption of such plans should be on advice of legal counsel or tax advisor.
 
    For further information regarding plan administration, custodial fees and
other details, investors should contact their Morgan Stanley Dean Witter
Financial Advisor or other Selected Broker-Dealer representative or the Transfer
Agent.
 
EXCHANGE PRIVILEGE
 
Shares of each Class may be exchanged for shares of the same Class of any other
Morgan Stanley Dean Witter Multi-Class Fund without the imposition of any
exchange fee. Shares may also be exchanged for shares of the following funds:
Morgan Stanley Dean Witter Short-Term U.S. Treasury Trust, Morgan Stanley Dean
Witter Limited Term Municipal Trust, Morgan Stanley Dean Witter Short-Term Bond
Fund and five Morgan Stanley Dean Witter Funds which are money market funds (the
"Exchange Funds"). Class A shares may also be exchanged for shares of Morgan
Stanley Dean Witter Multi-State Municipal Series Trust and Morgan Stanley Dean
Witter Hawaii Municipal Trust, which are Morgan Stanley Dean Witter Funds sold
with a front-end sales charge ("FSC Funds"). Exchanges may be made after the
shares of the Fund acquired by purchase (not by exchange or dividend
reinvestment) have been held for thirty days. There is no waiting period for
exchanges of shares acquired by exchange or dividend reinvestment.
 
    An exchange to another Morgan Stanley Dean Witter Multi-Class Fund, any FSC
Fund or any Exchange Fund that is not a money market fund is on the basis of the
next calculated net asset value per share of each fund after the exchange order
is received. When exchanging into a money market fund from the Fund, shares of
the Fund are redeemed out of the Fund at their next calculated net asset value
and the proceeds of the redemption are used to purchase shares of the money
market fund at their net asset value determined the following business day.
Subsequent exchanges between any of the Morgan Stanley Dean Witter Multi-Class
Funds, FSC Funds or any Exchange Fund that is not a money market fund can be
effected on the same basis.
 
    No CDSC is imposed at the time of any exchange of shares, although any
applicable CDSC will be imposed upon ultimate redemption. During the period of
time the shareholder remains in an Exchange Fund (calculated from the last day
of the month in which the Exchange Fund shares were acquired), the holding
period (for the purpose of determining the rate of the CDSC) is frozen. If those
shares are subsequently re-exchanged for shares of a Morgan Stanley Dean Witter
Multi-Class Fund, the holding period previously frozen when the first exchange
was made resumes on the last day of the month in which shares of a Morgan
Stanley Dean Witter Multi-Class Fund are reacquired. Thus, the CDSC is based
upon the time (calculated as described above) the shareholder was invested in
shares of a Morgan Stanley Dean Witter Multi-Class Fund (see "Purchase of Fund
Shares"). In the case of exchanges of Class A shares which are subject to a
CDSC, the holding period also includes the time (calculated as described above)
the shareholder was invested in shares of a FSC Fund. In the case of shares
exchanged into an Exchange Fund on or after April 23, 1990, upon a redemption of
shares which results in a CDSC being imposed, a credit (not to exceed the amount
of the CDSC) will be given in an amount equal to the Exchange Fund 12b-1
distribution fees incurred on or after that date which are attributable to those
shares. (Exchange Fund 12b-1 distribution fees are described in the prospectuses
for those funds.) Class B shares of the Fund acquired in exchange for Class B
shares of another Morgan Stanley Dean Witter Multi-Class Fund having a different
CDSC schedule than that of this Fund will be subject to the higher CDSC
schedule, even if such shares are subsequently re-exchanged for shares of the
fund with the lower CDSC schedule.
 
ADDITIONAL INFORMATION REGARDING EXCHANGES.  Purchases and exchanges should be
made for investment purposes only. A pattern of frequent exchanges may be deemed
by the Investment Manager to be abusive and contrary to the best interests of
the Fund's other shareholders and, at the Investment Manager's discretion, may
be limited by the Fund's refusal to accept additional purchases and/or exchanges
from the investor. Although the Fund does not have any specific definition of
what constitutes a pattern of frequent exchanges, and will consider all relevant
factors in determining whether a particular situation is abusive and contrary to
the best interests of the Fund and its other shareholders, investors should be
aware that the Fund and each of the other Morgan Stanley Dean Witter Funds may
in their discretion limit or otherwise restrict the number of times this
Exchange Privilege may be exercised by any investor. Any such restriction will
be made by the Fund on a prospective basis only, upon notice of the shareholder
not later than ten days following such shareholder's most recent exchange. Also,
the Exchange Privilege may be terminated or revised at any time by the Fund
and/or any of such Morgan Stanley Dean Witter Funds for which shares of the Fund
have been exchanged, upon such notice as may be required by applicable
regulatory agencies. Shareholders
 
                                                                              21
<PAGE>
maintaining margin accounts with DWR or another Selected Broker-Dealer are
referred to their Morgan Stanley Dean Witter Financial Advisor or other Selected
Broker-Dealer representative regarding restrictions on exchange of shares of the
Fund pledged in the margin account.
 
    The current prospectus for each fund describes its investment objective and
policies, and shareholders should obtain a copy and examine it carefully before
investing. Exchanges are subject to the minimum investment requirement of each
Class of shares and any other conditions imposed by each fund. In the case of a
shareholder holding a share certificate or certificates, no exchanges may be
made until all applicable share certificates have been received by the Transfer
Agent and deposited in the shareholder's account. An exchange will be treated
for federal income tax purposes the same as a repurchase or redemption of
shares, on which the shareholder may realize a capital gain or loss. However,
the ability to deduct capital losses on an exchange may be limited in situations
where there is an exchange of shares within ninety days after the shares are
purchased. The Exchange Privilege is only available in states where an exchange
may legally be made.
 
    If DWR or another Selected Broker-Dealer is the current dealer of record and
its account numbers are part of the account information, shareholders may
initiate an exchange of shares of the Fund for shares of any of the Morgan
Stanley Dean Witter Funds (for which the Exchange Privilege is available)
pursuant to this Exchange Privilege by contacting their Morgan Stanley Dean
Witter Financial Advisor or other Selected Broker-Dealer representative (no
Exchange Privilege Authorization Form is required). Other shareholders (and
those shareholders who are clients of DWR or other Selected Broker-Dealers but
who wish to make exchanges directly by writing or telephoning the Transfer
Agent) must complete and forward to the Transfer Agent an Exchange Privilege
Authorization Form, copies of which may be obtained from the Transfer Agent, to
initiate an exchange. If the Authorization Form is used, exchanges may be made
in writing or by contacting the Transfer Agent at (800) 869-NEWS (toll-free).
 
    The Fund will employ reasonable procedures to confirm that exchange
instructions communicated over the telephone are genuine. Such procedures may
include requiring various forms of personal identification such as name, mailing
address, social security or other tax identification number and DWR or other
Selected Broker-Dealer account number (if any). Telephone instructions may also
be recorded. If such procedures are not employed, the Fund may be liable for any
losses due to unauthorized or fraudulent instructions.
 
    Telephone exchange instructions will be accepted if received by the Transfer
Agent between 9:00 a.m. and 4:00 p.m., New York time, on any day the New York
Stock Exchange is open. Any shareholder wishing to make an exchange who has
previously filed an Exchange Privilege Authorization Form and who is unable to
reach the Fund by telephone should contact his or her Morgan Stanley Dean Witter
Financial Advisor or other Selected Broker-Dealer representative, if
appropriate, or make a written exchange request. Shareholders are advised that
during periods of drastic economic or market changes, it is possible that the
telephone exchange procedures may be difficult to implement, although this has
not been the experience with the Morgan Stanley Dean Witter Funds in the past.
 
    For further information regarding the Exchange Privilege, shareholders
should contact their Morgan Stanley Dean Witter Financial Advisor or other
Selected Broker-Dealer representative or the Transfer Agent.
 
REDEMPTIONS AND REPURCHASES
- --------------------------------------------------------------------------------
 
REDEMPTION.  Shares of each Class of the Fund can be redeemed for cash at any
time at the net asset value per share next determined less the amount of any
applicable CDSC in the case of Class A, Class B or Class C shares (see "Purchase
of Fund Shares"). If shares are held in a shareholder's account without a share
certificate, a written request for redemption to the Fund's Transfer Agent at
P.O. Box 983, Jersey City, NJ 07303 is required. If certificates are held by the
shareholder, the shares may be redeemed by surrendering the certificates with a
written request for redemption along with any additional documentation required
by the Transfer Agent.
 
REPURCHASE.  DWR and other Selected Broker-Dealers are authorized to repurchase
shares represented by a share certificate which is delivered to any of their
offices. Shares held in a shareholder's account without a share certificate may
also be repurchased by DWR and other Selected Broker-Dealers upon the telephonic
or telegraphic request of the shareholder. The repurchase price is the net asset
value next computed (see "Purchase of Fund Shares") after such repurchase order
is received by DWR and other Selected Broker-Dealers, reduced by any applicable
CDSC.
 
    The CDSC, if any, will be the only fee imposed by the Fund or the
Distributor. The offer by DWR and other Selected Broker-Dealers to repurchase
shares may be suspended without notice by them at any time. In that event,
shareholders may redeem their shares through the Fund's Transfer Agent as set
forth above under "Redemption."
 
PAYMENT FOR SHARES REDEEMED OR REPURCHASED.  Payment for shares presented for
repurchase or redemption will be made by check within seven days after receipt
by the Transfer Agent of the certificate and/or written request in good order.
Such payment may be postponed or the right of redemption suspended under unusual
circumstances,
 
22
<PAGE>
e.g., when normal trading is not taking place on the New York Stock Exchange. If
the shares to be redeemed have recently been purchased by check, payment of the
redemption proceeds may be delayed for the minimum time needed to verify that
the check used for investment has been honored (not more than fifteen days from
the time of receipt of the check by the Transfer Agent). Shareholders
maintaining margin accounts with DWR or another Selected Broker-Dealer are
referred to their Morgan Stanley Dean Witter Financial Advisor or other Selected
Broker-Dealer representative regarding restrictions on redemption of shares of
the Fund pledged in the margin account.
 
REINSTATEMENT PRIVILEGE.  A shareholder who has had his or her shares redeemed
or repurchased and has not previously exercised this reinstatement privilege
may, within 35 days after the date of the redemption or repurchase, reinstate
any portion or all of the proceeds of such redemption or repurchase in shares of
the Fund in the same Class from which such shares were redeemed or repurchased,
at their net asset value next determined after a reinstatement request, together
with the proceeds, is received by the Transfer Agent and receive a pro rata
credit for any CDSC paid in connection with such redemption or repurchase.
 
INVOLUNTARY REDEMPTION.  The Fund reserves the right to redeem, on sixty days'
notice and at net asset value, the shares (other than shares held in an
Individual Retirement Account or Custodial Account under Section 403(b)(7) of
the Internal Revenue Code) of any shareholder whose shares, due to redemptions
by the shareholder, have a value of less than $100 or such lesser amount as may
be fixed by the Trustees or, in the case of an account opened through
EasyInvest-SM-, if after twelve months the shareholder has invested less than
$1,000 in the account. However, before the Fund redeems such shares and sends
the proceeds to the shareholder it will notify the shareholder that the value of
the shares is less than the applicable amount and allow the shareholder sixty
days to make an additional investment in an amount which will increase the value
of the account to at least the applicable amount before the redemption is
processed. No CDSC will be imposed on any involuntary redemption.
 
DIVIDENDS, DISTRIBUTIONS AND TAXES
- --------------------------------------------------------------------------------
 
DIVIDENDS AND DISTRIBUTIONS.  The Fund declares dividends separately for each
Class of shares and intends to declare dividends from net investment income on
each day the New York Stock Exchange is open for business (see "Purchase of Fund
Shares"). The amount of the dividend declared may fluctuate from day to day.
Dividends are declared daily and paid monthly in additional shares of the Fund.
The Fund intends to distribute dividends from net short-term and long-term
capital gains, if any, at least once each year. The Fund may, however, elect to
retain all or a portion of any net long-term capital gains in any year.
 
    All dividends and any capital gains distributions will be paid in additional
shares of the same Class and automatically credited to the shareholder's account
without issuance of a share certificate unless the shareholder requests in
writing that all dividends or all dividends and distributions be paid in cash.
Shares acquired by dividend and distribution reinvestments will not be subject
to any front-end sales charge or CDSC. Class B shares acquired through dividend
and distribution reinvestments will become eligible for conversion to Class A
shares on a pro rata basis. Distributions paid on Class A and Class D shares
will be higher than for Class B and Class C shares because distribution fees
paid by Class B and Class C shares are higher. (See "Shareholder
Services--Automatic Investment of Dividends and Distributions".)
 
TAXES.  Because the Fund intends to distribute all of its net investment income
and capital gains to shareholders and otherwise continue to qualify as a
regulated investment company under Subchapter M of the Internal Revenue Code, it
is not expected that the Fund will be required to pay any federal income tax.
Shareholders who are required to pay taxes on their income will normally have to
pay federal income taxes, and any state and local income taxes, on the dividends
and distributions they receive from the Fund. Such dividends and distributions,
to the extent that they are derived from net investment income or net short-term
capital gains, are taxable to the shareholder as ordinary dividend income
regardless of whether the shareholder receives such payments in additional
shares or in cash. Any dividends declared in the last quarter of any calendar
year which are paid to shareholders of record in such period in the following
year prior to February 1 will be deemed, for tax purposes, to have been received
by the shareholder in the prior year.
 
    Long-term and short-term capital gains may be generated by the sale of
portfolio securities by the Fund. Distributions of net long-term capital gains,
if any, are taxable to shareholders as long-term capital gains regardless of how
long a shareholder has held the Fund's shares and regardless of whether the
distribution is received in additional shares or in cash. After the end of the
calendar year, shareholders will receive full information on their dividends and
capital gains distributions for tax purposes, including information as to the
portion taxable as ordinary income and the portion taxable as mid-term and
long-term capital gains. Shareholders will also be notified if their
proportionate share of long-term capital gains distributions that are eligible
for a reduced rate of tax under the Taxpayer Relief Act of 1997.
 
    The Fund may at times make payments from sources other than income or net
capital gains. Payments from such sources would, in effect, represent a return
of a portion of
 
                                                                              23
<PAGE>
each shareholder's investment. All, or a portion, of such payments would not be
taxable to shareholders.
 
    To avoid being subject to a 31% federal backup withholding tax on taxable
dividends, capital gains distributions and the proceeds of redemptions and
repurchases, shareholders' taxpayer identification numbers must be furnished and
certified as to accuracy. The foregoing discussion relates solely to the federal
income tax consequences of an investment in the Fund. Distributions may also be
subject to state and local taxes; therefore, each shareholder is advised to
consult his or her own tax advisor.
 
PERFORMANCE INFORMATION
- --------------------------------------------------------------------------------
 
From time to time the Fund may quote its "yield" and/or its "total return" in
advertisements and sales literature. These figures are computed separately for
Class A, Class B, Class C and Class D shares. Both the yield and the total
return of the Fund are based on historical earnings and are not intended to
indicate future performance. The yield of each Class of the Fund is computed by
dividing the Class's net investment income over a 30-day period by an average
value (using the average number of shares entitled to receive dividends and the
maximum offering price per share at the end of the period), all in accordance
with applicable regulatory requirements. Such amount is compounded for six
months and then annualized for a twelve-month period to derive the Fund's yield
for each Class.
 
    The "average annual total return" of the Fund refers to a figure reflecting
the average annualized percentage increase (or decrease) in the value of an
initial investment in a Class of the Fund of $1,000 over periods of one, five
and ten years, or over the life of the Fund, if less than any of the foregoing.
Average annual total return reflects all income earned by the Fund, any
appreciation or depreciation of the Fund's assets, all expenses incurred by the
applicable Class and all sales charges incurred by shareholders, for the stated
periods. It also assumes reinvestment of all dividends and distributions paid by
the Fund.
 
    In addition to the foregoing, the Fund may advertise its total return for
each Class over different periods of time by means of aggregate, average,
year-by-year or other types of total return figures. The Fund may also advertise
the growth of hypothetical investments of $10,000, $50,000 and $100,000 in each
Class of shares of the Fund. Such calculations may or may not reflect the
deduction of any sales charge which, if reflected, would reduce the performance
quoted. The Fund from time to time may also advertise its performance relative
to certain performance rankings and indexes compiled by independent
organizations, such as mutual fund performance rankings of Lipper Analytical
Services, Inc.
 
ADDITIONAL INFORMATION
- --------------------------------------------------------------------------------
 
VOTING RIGHTS.  All shares of beneficial interest of the Fund are of $0.01 par
value and are equal as to earnings, assets and voting privileges except that
each Class will have exclusive voting privileges with respect to matters
relating to distribution expenses borne solely by such Class or any other matter
in which the interests of one Class differ from the interests of any other
Class. In addition, Class B shareholders will have the right to vote on any
proposed material increase in Class A's expenses, if such proposal is submitted
separately to Class A shareholders. Also, as discussed herein, Class A, Class B
and Class C bear the expenses related to the distribution of their respective
shares.
 
    The Fund is not required to hold Annual Meetings of Shareholders and, in
ordinary circumstances, the Fund does not intend to hold such meetings. The
Trustees may call Special Meetings of Shareholders for action by shareholder
vote as may be required by the Act or the Declaration of Trust.
 
    Under Massachusetts law, shareholders of a business trust may, under certain
circumstances, be held personally liable as partners for obligations of the
Fund. However, the Declaration of Trust contains an express disclaimer of
shareholder liability for acts or obligations of the Fund, requires that Fund
obligations include such disclaimer, and provides for indemnification out of the
Fund's property for any shareholder held personally liable for the obligations
of the Fund. Thus, the risk of a shareholder incurring financial loss on account
of shareholder liability is limited to circumstances in which the Fund itself
would be unable to meet its obligations. Given the above limitations on
shareholder personal liability and the nature of the Fund's assets and
operations, the possibility of the Fund being unable to meet its obligations is
remote and, in the opinion of Massachusetts counsel to the Fund, the risk to
Fund shareholders of personal liability is remote.
 
CODE OF ETHICS.  Directors, officers and employees of MSDW Advisors, MSDW
Services and MSDW Distributors are subject to a strict Code of Ethics adopted by
those companies. The Code of Ethics is intended to ensure that the interests of
shareholders and other clients are placed ahead of any personal interest, that
no undue personal benefit is obtained from a person's employment activities and
that actual and potential conflicts of interest are avoided. To achieve these
goals and comply with regulatory requirements, the Code of Ethics requires,
among other things, that personal securities transactions by employees of the
companies be subject to an advance clearance process to
 
24
<PAGE>
monitor that no Morgan Stanley Dean Witter Fund is engaged at the same time in a
purchase or sale of the same security. The Code of Ethics bans the purchase of
securities in an initial public offering, and also prohibits engaging in futures
and options transactions and profiting on short-term trading (that is, a
purchase within sixty days of a sale or a sale within sixty days of a purchase)
of a security. In addition, investment personnel may not purchase or sell a
security for their personal account within thirty days before or after any
transaction in any Morgan Stanley Dean Witter Fund managed by them. Any
violations of the Code of Ethics are subject to sanctions, including reprimand,
demotion or suspension or termination of employment. The Code of Ethics comports
with regulatory requirements and the recommendations in the 1994 report by the
Investment Company Institute Advisory Group on Personal Investing.
 
MASTER/FEEDER CONVERSION.  The Fund reserves the right to seek to achieve its
investment objective by investing all of its investable assets in a diversified,
open-end management investment company having the same investment objective and
policies and substantially the same investment restrictions as those applicable
to the Fund.
 
SHAREHOLDER INQUIRIES.  All inquiries regarding the Fund should be directed to
the Fund at the telephone numbers or address set forth on the front cover of
this Prospectus.
 
                                                                              25
<PAGE>
MORGAN STANLEY DEAN WITTER
INTERMEDIATE INCOME SECURITIES
TWO WORLD TRADE CENTER
NEW YORK, NEW YORK 10048
 
TRUSTEES
Michael Bozic
Charles A. Fiumefreddo
Edwin J. Garn
John R. Haire
Wayne E. Hedien
Dr. Manuel H. Johnson
Michael E. Nugent
Philip J. Purcell
John L. Schroeder
 
OFFICERS
Charles A. Fiumefreddo
Chairman and Chief Executive Officer
Barry Fink
Vice President, Secretary and
General Counsel
Rochelle G. Siegel
Vice President
Thomas F. Caloia
Treasurer
 
CUSTODIAN
The Bank of New York
90 Washington Street
New York, New York 10286
 
TRANSFER AGENT AND
DIVIDEND DISBURSING AGENT
Morgan Stanley Dean Witter Trust FSB
Harborside Financial Center
Plaza Two
Jersey City, New Jersey 07311
 
INDEPENDENT ACCOUNTANTS
PricewaterhouseCoopers LLP
1177 Avenue of the Americas
New York, New York 10036
 
INVESTMENT MANAGER
Morgan Stanley Dean Witter Advisors Inc.


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