MFS SERIES TRUST X
497, 2000-09-29
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                          MFS[RegTM] NEW ENDEAVOR FUND

           Supplement dated October 1, 2000 to the Current Prospectus

This Supplement describes the fund's class I shares, and it supplements certain
information in the fund's Prospectus dated October 1, 2000. The caption headings
used in this Supplement correspond with the caption headings used in the
Prospectus.

You may purchase class I shares only if you are an eligible institutional
investor, as described under the caption "Description of Share Classes" below.


1.   RISK RETURN SUMMARY

     Performance Table. The performance table is not included because the fund
     has not had a full year of operation.


2.   EXPENSE SUMMARY

     Expense Table. The "Expense Table" describes the fees and expenses that you
     may pay when you buy, redeem and hold shares of the fund. The table is
     supplemented as follows:

Annual Fund Operating Expenses (expenses that are deducted from fund assets)

<TABLE>
<S>                                                                      <C>
      Management Fees.......................................             0.90%
      Distribution and Service (12b-1) Fees.................             0.00%
      Other Expenses........................................             3.88%
                                                                         -----
      Total Annual Fund Operating Expenses..................             4.78%
           Expense Reimbursement(1).........................            (3.58)%
                                                                        -------
           Net Expenses(2)..................................             1.20%
</TABLE>

-----------------------

(1)    "Other Expenses" for the fund are based on estimates for the fund's
       current fiscal year. MFS has contractually agreed to bear the fund's
       expenses subject to reimbursement, such that "Other Expenses", after
       taking into account the expense offset arrangement described below, do
       not exceed 0.30%. These contractual fee arrangements will continue until
       at least October 1, 2001, absent an earlier modification approved by the
       board of trustees which oversees the fund.

(2)    The fund has an expense offset arrangement which reduces the fund's
       custodian fee based upon the amount of cash maintained by the fund with
       its custodian and dividend disbursing agent. The fund may enter into
       other similar arrangements and directed brokerage arrangements, which
       would also have the effect of reducing the fund's expenses. Any such fee
       reductions are not reflected in the table. Had such fee reductions been
       taken into account, "Net Expenses" would be lower.

Example of Expenses. The "Example of Expenses" table is intended to help you
compare the cost of investing in the fund with the cost of investing in other
mutual funds. The examples assume that:

     o    You invest $10,000 in the fund for the time periods indicated and you
          redeem your shares at the end of the timer periods;

     o    Your investment has a 5% return each year and dividends and other
          distributions are reinvested; and

     o    The fund's operating expenses remain the same, except that the fund's
          total operating expenses are assumed to be the fund's "Net Expenses"
          for the first year, and the fund's "Total Annual Operating Fund
          Expenses" for subsequent years (see Expense Table).

The table is supplemented as follows:

<TABLE>
<CAPTION>
                Share Class           Year 1             Year 3
                -----------           ------             ------
             <S>                       <C>                <C>
             Class I shares            $123               $386
</TABLE>


                                      -1-
<PAGE>


3.   DESCRIPTION OF SHARE CLASS

The "Description of Share Classes" is supplemented as follows:

If you are an eligible institutional investor (as described below), you may
purchase class I shares at net asset value without an initial sales charge or
CDSC upon redemption. Class I shares do not have annual distribution and service
fees, and do not convert to any other class of shares of the fund.

The following eligible institutional investors may purchase class I shares:

     o    certain retirement plans established for the benefit of employees of
          MFS and employees of MFS' affiliates;

     o    any fund distributed by MFD, if the fund seeks to achieve its
          investment objective by investing primarily in shares of the fund and
          other MFS funds;

     o    any retirement plan, endowment or foundation which:

          >    purchases shares directly through MFD (rather than through a
               third party broker or dealer or other financial adviser);

          >    has, at the time of purchase of class I shares, aggregate assets
               of at least $100 million; and

          >    invests at least $10 million in class I shares of the fund either
               alone or in combination with investments in class I shares of
               other MFS Funds (additional investments may be made in any
               amount).

     MFD may accept purchases from smaller plans, endowments or foundations or
     in smaller amounts if it believes, in its sole discretion, that such
     entity's aggregate assets will equal or exceed $100 million, or that such
     entity will make additional investments which will cause its total
     investment to equal or exceed $10 million, within a reasonable period of
     time;

     o    bank trust departments or law firms acting as trustee or manager for
          trust accounts which, on behalf of their clients (i) initially invest
          at least $100,000 in class I shares of the fund or (ii) have, at the
          time of purchase of class I shares, aggregate assets of at least $10
          million invested in class I shares of the fund either alone or in
          combination with investments in class I shares of other MFS Funds. MFD
          may accept purchases that do not meet these dollar qualification
          requirements if it believes, in its sole discretion, that these
          requirements will be met within a reasonable period of time.
          Additional investments may be made in any amount; and

     o    certain retirement plans offered, administered or sponsored by
          insurance companies, provided that these plans and insurance companies
          meet certain criteria established by MFD from time to time.

In no event will the fund, MFS, MFD or any of their affiliates pay any sales
commissions or compensation to any third party in connection with the sale of
class I shares. The payment of any such sales commission or compensation would,
under the fund's policies, disqualify the purchaser as an eligible investor in
class I shares.


4.   HOW TO PURCHASE, EXCHANGE AND REDEEM SHARES

The discussion of "How to Purchase, Exchange and Redeem Shares" is supplemented
as follows:

You may purchase, redeem and exchange class I shares only through your MFD
representative or by contacting MFSC (see the back cover of the Prospectus for
address and phone number). You may exchange your class I shares for class I
shares of another MFS Fund (if you are eligible to purchase them) and for shares
of the MFS Money Market Fund at net asset value.

                 The date of this Supplement is October 1, 2000.


                                       -2-
<PAGE>

--------------------------------
  MFS[RegTM] NEW ENDEAVOR FUND
--------------------------------

  O C T O B E R  1 ,  2 0 0 0


                                                                      Prospectus

                                                                  Class A Shares
                                                                  Class B Shares
                                                                  Class C Shares
--------------------------------------------------------------------------------

This Prospectus describes the MFS[RegTM] New Endeavor Fund. The fund's
investment objective is capital appreciation.

This prospectus describes three classes of shares for the fund. Currently, only
Class A shares are available for purchase. These Class A shares are only
available for purchase at net asset value and may only be sold to residents of
Massachusetts who are:

o  employees (or certain relatives of employees) of Massachusetts Financial
   Services Company (referred to as MFS or the adviser) and its affiliates; or

o  members of the governing boards of the various funds sponsored by MFS.

The Securities and Exchange Commission has not approved or disapproved the
fund's shares or determined whether this prospectus is accurate or complete.
Anyone who tells you otherwise is committing a crime.
<PAGE>

---------------------
  TABLE OF CONTENTS
---------------------

<TABLE>
<CAPTION>
Page
<S>    <C>                                                       <C>
I      Risk Return Summary ...................................... 1
II     Expense Summary .......................................... 5
III    Certain Investment Strategies and Risks .................. 7
IV     Management of the Fund ................................... 8
V      Description of Share Classes ............................. 9
VI     How to Purchase, Exchange and Redeem Shares ..............13
VII    Investor Services and Programs ...........................17
VIII   Other Information ........................................19
       Appendix A--Investment Techniques and Practices ..........A-1
</TABLE>

<PAGE>

--------------------------
  I  RISK RETURN SUMMARY
--------------------------

>  Investment Objective

   The fund's investment objective is capital appreciation. The fund's objective
   may be changed without shareholder approval.

>  Principal Investment Policies

   The fund invests, under normal market conditions, at least 65% of its total
   assets in equity securities of emerging growth companies. Equity securities
   include common stocks and related securities, such as preferred stocks,
   convertible securities and depositary receipts for those securities. Emerging
   growth companies are companies which the fund's investment adviser,
   Massachusetts Financial Services Company (referred to as MFS or the adviser),
   believes offer superior prospects for growth and are either:

   o early in their life cycle but which have the potential to become major
     enterprises, or

   o are major enterprises whose rates of earnings growth are expected to
     accelerate because of special factors, such as rejuvenated management, new
     products, changes in consumer demand, or basic changes in the economic
     environment.

   Emerging growth companies may be of any size, and MFS would expect these
   companies to have products, technologies, management, markets and
   opportunities which will facilitate earnings growth over time, that is well
   above the growth rate of the overall economy, and the rate of inflation. The
   fund's investments in emerging growth companies may include securities listed
   on a securities exchange or traded in the over-the-counter markets.

   The fund is flexibly managed with the ability to invest in issuers of all
   market capitalizations and industry focus.

   Consistent with the fund's principal investment policies described above, the
   fund may invest in foreign securities (including emerging markets securities)
   and may have exposure to foreign currencies through its investment in these
   securities.

   The fund may engage in short sales where the fund borrows a security it does
   not own and then sells it in anticipation of a fall in the security's price.
   The fund must replace the security it borrowed by purchasing the security at
   its market value at the time of replacement. The fund may also engage in
   short sales "against the box" where the fund owns or has the right to obtain
   at no additional cost, the securities that are sold short.

   MFS uses a bottom-up, as opposed to a top-down, investment style in managing
   the equity-oriented funds (such as the fund) it advises. This means that
   securities are selected based upon fundamental analysis (such as an analysis
   of earnings, cash flows, competitive position and management's abilities)
   performed by the fund's portfolio manager and MFS' large group of equity
   research analysts.


                                       1
<PAGE>

   The fund may engage in active and frequent trading to achieve its principal
   investment strategies.

>  Principal Risks of an Investment

   The principal risks of investing in the fund and the circumstances reasonably
   likely to cause the value of your investment in the fund to decline are
   described below. The share price of the fund generally changes daily based on
   market conditions and other factors. Please note that there are many
   circumstances which could cause the value of your investment in the fund to
   decline, and which could prevent the fund from achieving its objective, that
   are not described here.

   The principal risks of investing in the fund are:

   o Market Risk: This is the risk that the price of a security held by the fund
     will fall due to changing economic, political or market conditions or
     disappointing earnings results.

   o Company Risk: Prices of securities react to the economic condition of the
     company that issued the security. The fund's equity investments in an
     issuer may rise and fall based on the issuer's actual and anticipated
     earnings, changes in management and the potential for takeovers and
     acquisitions.

   o Emerging Growth Companies Risk: Investments in emerging growth companies
     may be subject to more abrupt or erratic market movements and may involve
     greater risks than investments in other companies. Emerging growth
     companies often:

     > have limited product lines, markets and financial resources

     > are dependent on management by one or a few key individuals

     > have shares which suffer steeper than average price declines after
       disappointing earnings reports and are more difficult to sell at
       satisfactory prices

   o Over-the-Counter Risk: Over-the-counter (OTC) transactions involve risks in
     addition to those associated with transactions in securities traded on
     exchanges. OTC listed companies may have limited product lines, markets or
     financial resources. Many OTC stocks trade less frequently and in smaller
     volume than exchange listed stocks. The values of these stocks may be more
     volatile than exchange listed stocks, and the fund may experience
     difficulty in purchasing or selling these securities at a fair price.

   o Short Sales Risk: The fund will suffer a loss if it sells a security short
     and the value of the security rises rather than falls. Because the fund
     must purchase the security it borrowed in a short sale at prevailing market
     rates, the potential loss may be greater for a short sale than for a short
     sale "against the box".

   o Foreign Securities Risk: Investing in foreign securities involves risks
     relating to political, social and economic developments abroad, as well as
     risks resulting from the differences between the regulations to which U.S.
     and foreign issuers and markets are subject:


                                       2
<PAGE>

     > These risks may include the seizure by the government of company assets,
       excessive taxation, withholding taxes on dividends and interest,
       limitations on the use or transfer of portfolio assets, and political or
       social instability.

     > Enforcing legal rights may be difficult, costly and slow in foreign
       countries, and there may be special problems enforcing claims against
       foreign governments.

     > Foreign companies may not be subject to accounting standards or
       governmental supervision comparable to U.S. companies, and there may be
       less public information about their operations.

     > Foreign markets may be less liquid and more volatile than U.S. markets.

     > Foreign securities often trade in currencies other than the U.S. dollar,
       and the fund may directly hold foreign currencies and purchase and sell
       foreign currencies through forward exchange contracts. Changes in
       currency exchange rates will affect the fund's net asset value, the value
       of dividends and interest earned, and gains and losses realized on the
       sale of securities. An increase in the strength of the U.S. dollar
       relative to these other currencies may cause the value of the fund to
       decline. Certain foreign currencies may be particularly volatile, and
       foreign governments may intervene in the currency markets, causing a
       decline in value or liquidity in the fund's foreign currency holdings. By
       entering into forward foreign currency exchange contracts, the fund may
       be required to forego the benefits of advantageous changes in exchange
       rates and, in the case of forward contracts entered into for the purpose
       of increasing return, the fund may sustain losses which will reduce its
       gross income. Forward foreign currency exchange contracts involve the
       risk that the party with which the fund enters the contract may fail to
       perform its obligations to the fund.

   o Emerging Markets Risk: Emerging markets are generally defined as countries
     in the initial stages of their industrialization cycles with low per capita
     income. All of the risks of investing in foreign securities described above
     are heightened by investing in emerging markets countries. The markets of
     emerging markets countries are generally more volatile than the markets of
     developed countries with more mature economies.

   o Effect of IPOs: The fund may participate in the initial public offering
     ("IPO") market, and a significant portion of the fund's returns may be
     attributable to its investment in IPO's which may have a magnified
     investment performance impact during the periods when the fund has a small
     asset base. Like any past performance there is no assurance that, as a
     fund's assets grow, it will continue to experience substantially similar
     performance by investment in IPOs.


                                       3
<PAGE>

   o Active or Frequent Trading Risk: The fund may engage in active and frequent
     trading to achieve its principal investment strategies. This may result in
     the realization and distribution to shareholders of higher capital gains as
     compared to a fund with less active trading policies, which would increase
     your tax liability. Frequent trading also increases transaction costs,
     which could detract from the fund's performance.

   o As with any mutual fund, you could lose money on your investment in the
     fund.

   An investment in the fund is not a bank deposit and is not insured or
   guaranteed by the Federal Deposit Insurance Corporation or any other
   government agency.


>  Bar Chart and Performance Table

   The bar chart and performance table are not included because the fund has not
   had a full calendar year of investment operations.

                                       4
<PAGE>

------------------------
  II   EXPENSE SUMMARY
------------------------


>  Expense Table

   This table describes the fees and expenses that you may pay when you buy,
   redeem and hold shares of the fund.


   Shareholder Fees (fees paid directly from your investment):
   .............................................................................

<TABLE>
<CAPTION>
                                                              Class A         Class B    Class C
    <S>                                                      <C>             <C>           <C>
    Maximum Sales Charge (Load) Imposed on
    Purchases (as a percentage of offering price) .........  5.75%              0.00%      0.00%

    Maximum Deferred Sales Charge (Load) (as a
    percentage of original purchase price or
    redemption proceeds, whichever is less) ...............  See Below(1)       4.00%      1.00%
</TABLE>

   Annual Fund Operating Expenses (expenses that are deducted from fund assets):
   .............................................................................

<TABLE>
<S>                                                         <C>           <C>          <C>
    Management Fees ..................................       0.90%         0.90%        0.90%
    Distribution and Service (12b-1) Fees(2) .........       0.35%         1.00%        1.00%
    Other Expenses ...................................       3.88%         3.88%        3.88%
                                                            -----         -----        -----
    Total Annual Fund Operating Expenses .............       5.13%         5.78%        5.78%
    Expense Reimbursement(3) .........................      (3.58)%       (3.58)%      (3.58)%
                                                            -----         -----        -----
    Net Expenses(4) ..................................       1.55%         2.20%        2.20%
</TABLE>

     --------
    (1) An initial sales charge will not be deducted from your purchase if you
        buy $1 million or more of class A shares, or if you are investing
        through a retirement plan and your class A purchase meets certain
        requirements. However, in this case, a contingent deferred sales charge
        (referred to as a CDSC) of 1% may be deducted from your redemption
        proceeds if you redeem your investment within 12 months.
    (2) The fund adopted a distribution plan under Rule 12b-1 that permits it
        to pay marketing and other fees to support the sale and distribution of
        class A, B and C shares and the services provided to you by your
        financial adviser (referred to as distribution and service fees).
    (3) "Other Expenses" for the fund are based on estimates for the fund's
        current fiscal year. MFS has contractually agreed to bear the fund's
        expenses subject to reimbursement, such that "Other Expenses", after
        taking into account the expense offset arrangement described below, do
        not exceed 0.30%. This contractual fee arrangement will continue until
        at least October 1, 2001, absent an earlier modification approved by
        the board of trustees which oversees the fund.
    (4) The fund has an expense offset arrangement which reduces the fund's
        custodian fee based upon the amount of cash maintained by the fund with
        its custodian and dividend disbursing agent. The fund may enter into
        other similar arrangements and directed brokerage arrangements, which
        would also have the effect of reducing the fund's expenses. "Other
        Expenses" do not take into account these expense reductions, and
        therefore are higher than the actual expenses of the fund. Had these
        fee reductions been taken into account, "Net Expenses" would be lower.


                                       5
<PAGE>


>  Example of Expenses

   These examples are intended to help you compare the cost of investing in the
   fund with the cost of investing in other mutual funds.

   The examples assume that:
   o You invest $10,000 in the fund for the time periods indicated and you
     redeem your shares at the end of the time periods;
   o Your investment has a 5% return each year and dividends and other
     distributions are reinvested; and
   o The fund's operating expenses remain the same except that the fund's total
     operating expenses are assumed to be the fund's "Net Expenses" for the
     first year, and the fund's "Total Annual Fund Operating Expenses" for
     subsequent years (see Expense Table).

   Although your actual costs may be higher or lower, under these assumptions
   your costs would be:


<TABLE>
<CAPTION>
     Share Class                                    Year 1        Year 3
-----------------------------------------------   ----------   ------------
     <S>                                           <C>          <C>
     Class A shares                                $724         $1,723
     Class B shares
       Assuming redemption at end of period         623          1,702
       Assuming no redemption                       223          1,402
     Class C shares
       Assuming redemption at end of period         323          1,402
       Assuming no redemption                       223          1,402
</TABLE>

                                       6
<PAGE>

------------------------------------------------
  III  CERTAIN INVESTMENT STRATEGIES AND RISKS
------------------------------------------------


>  Further Information on Investment Strategies and Risks

   The fund may invest in various types of securities and engage in various
   investment techniques and practices which are not the principal focus of the
   fund and therefore are not described in this Prospectus. The types of
   securities and investment techniques and practices in which the fund may
   engage, including the principal investment techniques and practices described
   above, are identified in Appendix A to this Prospectus, and are discussed,
   together with their risks, in the fund's Statement of Additional Information
   (referred to as the SAI), which you may obtain by contacting MFS Service
   Center, Inc. (see back cover for address and phone number).

>  Temporary Defensive Policies

   In addition, the fund may depart from its principal investment strategies by
   temporarily investing for defensive purposes when adverse market, economic or
   political conditions exist. While the fund invests defensively, it may not be
   able to pursue its investment objective. The fund's defensive investment
   position may not be effective in protecting its value.


                                       7
<PAGE>

-------------------------------
  IV   MANAGEMENT OF THE FUND
-------------------------------


>  Investment Adviser

   Massachusetts Financial Services Company (referred to as MFS or the adviser)
   is the fund's investment adviser. MFS is America's oldest mutual fund
   organization. MFS and its predecessor organizations have a history of money
   management dating from 1924 and the founding of the first mutual fund,
   Massachusetts Investors Trust. Net assets under the management of the MFS
   organization were approximately 163.20 billion as of August 31, 2000. MFS is
   located at 500 Boylston Street, Boston, MA 02116. MFS provides investment
   management and related administrative services and facilities to the fund
   (including portfolio management and trade execution). For these services, MFS
   is entitled to an annual management fee as set forth in the Expense Summary.


>  Portfolio Manager

   The fund's portfolio manager is Brian E. Stack, a Senior Vice President of
   MFS. Mr. Stack has been employed in the investment management area of MFS
   since 1993 and has been the portfolio manager of the fund since its
   inception.


>  Administrator

   MFS provides the fund with certain financial, legal, compliance, shareholder
   communications and other administrative services. MFS is reimbursed by the
   fund for a portion of the costs it incurs in providing these services.


>  Distributor

   MFS Fund Distributors, Inc. (referred to as MFD), a wholly owned subsidiary
   of MFS, is the distributor of shares of the fund.


>  Shareholder Servicing Agent

   MFS Service Center, Inc. (referred to as MFSC), a wholly owned subsidiary of
   MFS, performs transfer agency and certain other services for the fund, for
   which it receives compensation from the fund.


                                       8
<PAGE>

------------------------------------
  V   DESCRIPTION OF SHARE CLASSES
------------------------------------


   The fund offers class A, B and C shares through this prospectus. The fund
   also offers an additional class of shares, class I shares, exclusively to
   certain institutional investors. Class I shares are made available through a
   separate prospectus supplement provided to institutional investors eligible
   to purchase them.

>  Sales Charges

   You may be subject to an initial sales charge when you purchase, or a CDSC
   when you redeem, class A, B or C shares. These sales charges are described
   below. In certain circumstances, these sales charges are waived. These
   circumstances are described in the SAI. Special considerations concerning the
   calculation of the CDSC that apply to each of these classes of shares are
   described below under the heading "Calculation of CDSC."

   If you purchase your fund shares through a financial adviser (such as a
   broker or bank), the adviser may receive commissions or other concessions
   which are paid from various sources, such as from the sales charges and
   distribution and service fees, or from MFS or MFD. These commissions and
   concessions are described in the SAI.


>  Class A Shares

   You may purchase class A shares at net asset value plus an initial sales
   charge (referred to as the offering price), but in some cases you may
   purchase class A shares without an initial sales charge but subject to a 1%
   CDSC upon redemption within one year. Class A shares have annual distribution
   and service fees up to a maximum of 0.35% of net assets annually.

   Purchases Subject to an Initial Sales Charge. The amount of the initial sales
   charge you pay when you buy class A shares differs depending upon the amount
   you invest, as follows:


<TABLE>
<CAPTION>
                                                 Sales Charge* as Percentage of:
                                                 -------------------------------
                                                      Offering    Net Amount
    Amount of Purchase                                  Price      Invested
<S>                                                       <C>        <C>
    Less than $50,000                                     5.75%      6.10
    $50,000 but less than $100,000                        4.75       4.99
    $100,000 but less than $250,000                       4.00       4.17
    $250,000 but less than $500,000                       2.95       3.04
    $500,000 but less than $1,000,000                     2.20       2.25
    $1,000,000 or more                                   None**     None**
</TABLE>

    --------
    *     Because of rounding in the calculation of offering price, actual
          sales charges you pay may be more or less than those calculated using
          these percentages.
    **    A 1% CDSC will apply to such purchases, as discussed below.

                                       9
<PAGE>

   Purchases Subject to a CDSC (but not an initial sales charge). You pay no
   initial sales charge when you invest $1 million or more in class A shares.
   However, a CDSC of 1% will be deducted from your redemption proceeds if you
   redeem within 12 months of your purchase.

   In addition, purchases made under the following four categories are not
   subject to an initial sales charge; however, a CDSC of 1% will be deducted
   from redemption proceeds if the redemption is made within 12 months of
   purchase:

   o Investments in class A shares by certain retirement plans subject to the
     Employee Retirement Income Security Act of 1974, as amended (referred to as
     ERISA), if, prior to July 1, 1996

     > the plan had established an account with MFSC; and

     > the sponsoring organization had demonstrated to the satisfaction of MFD
       that either;

       + the employer had at least 25 employees; or

       + the total purchases by the retirement plan of class A shares of the MFS
         Family of Funds (referred to as the MFS funds) would be in the amount
         of at least $250,000 within a reasonable period of time, as determined
         by MFD in its sole discretion.

   o Investments in class A shares by certain retirement plans subject to ERISA,
     if

     > the retirement plan and/or sponsoring organization participates in the
       MFS Corporate Plan Services 401(k) Program or any similar recordkeeping
       system made available by MFSC (referred to as the MFS participant
       recordkeeping system);

     > the plan establishes an account with MFSC on or after July 1, 1996;

     > the total purchases by the retirement plan (or by multiple plans
       maintained by the same plan sponsor) of class A shares of the MFS funds
       will be in the amount of at least $500,000 within a reasonable period of
       time, as determined by MFD in its sole discretion; and

   o Investments in class A shares by certain retirement plans subject to
     ERISA, if

     > the plan establishes an account with MFSC on or after July 1, 1996; and

     > the plan has, at the time of purchase, either alone or in aggregate with
       other plans maintained by the same plan sponsor) a market value of
       $500,000 or more invested in shares of any class or classes of the MFS
       funds.

       The retirement plans will qualify under this category only if the plans
       or their sponsoring organization informs MFSC prior to the purchases that
       the plans have a market value of $500,000 or more invested in shares of
       any class or classes of the MFS funds; MFSC has no obligation
       independently to determine whether such plans qualify under this
       category; and


                                       10
<PAGE>

   o Investments in class A shares by certain retirement plans subject to
     ERISA, if

     > the plan established an account with MFSC between July 1, 1997 and
       December 31, 1999;

     > the plan records are maintained on a pooled basis by MFSC; and

     > the sponsoring organization demonstrates to the satisfaction of MFD that,
       at the time of purchase, the employer has at least 200 eligible employees
       and the plan has aggregate assets of at least $2,000,000.

>  Class B Shares

   You may purchase class B shares at net asset value without an initial sales
   charge, but if you redeem your shares within the first six years you may be
   subject to a CDSC (declining from 4.00% during the first year to 0% after six
   years). Class B shares have annual distribution and service fees up to a
   maximum of 1.00% of net assets annually.

   The CDSC is imposed according to the following schedule:


<TABLE>
<CAPTION>
                                                   Contingent Deferred
     Year of Redemption After Purchase                Sales Charge
     -----------------------------------------------------------------
     <S>                                                   <C>
     First                                                 4%
     Second                                                4%
     Third                                                 3%
     Fourth                                                3%
     Fifth                                                 2%
     Sixth                                                 1%
     Seventh and following                                 0%
</TABLE>

   If you hold class B shares for approximately eight years, they will convert
   to class A shares of the fund. All class B shares you purchased through the
   reinvestment of dividends and distributions will be held in a separate
   sub-account. Each time any class B shares in your account convert to class A
   shares, a proportionate number of the class B shares in the sub-account will
   also convert to class A shares.

>  Class C Shares

   You may purchase class C shares at net asset value without an initial sales
   charge, but if you redeem your shares within the first year you may be
   subject to a CDSC of 1.00%. Class C shares have annual distribution and
   service fees up to a maximum of 1.00% of net assets annually. Class C shares
   do not convert to any other class of shares of the fund.


                                       11
<PAGE>


>  Calculation of CDSC

   As discussed above, certain investments in class A, B and C shares will be
   subject to a CDSC. Three different aging schedules apply to the calculation
   of the CDSC:

   Purchases of class A shares made on any day during a calendar month will age
   one month on the last day of the month, and each subsequent month.

   Purchases of class C shares, and purchases of class B shares on or after
   January 1, 1993, made on any day during a calendar month will age one year at
   the close of business on the last day of that month in the following calendar
   year, and each subsequent year.

   Purchases of class B shares prior to January 1, 1993 made on any day during a
   calendar year will age one year at the close of business on December 31 of
   that year, and each subsequent year.

   No CDSC is assessed on the value of your account represented by appreciation
   or additional shares acquired through the automatic reinvestment of dividends
   or capital gain distributions. Therefore, when you redeem your shares, only
   the value of the shares in excess of these amounts (i.e., your direct
   investment) is subject to a CDSC.

   The CDSC will be applied in a manner that results in the CDSC being imposed
   at the lowest possible rate, which means that the CDSC will be applied
   against the lesser of your direct investment or the total cost of your
   shares. The applicability of a CDSC will not be affected by exchanges or
   transfers of registration, except as described in the SAI.


>  Distribution and Service Fees

   The fund has adopted a plan under Rule 12b-1 that permits it to pay marketing
   and other fees to support the sale and distribution of class A, B and C
   shares and the services provided to you by your financial adviser. These
   annual distribution and service fees may equal up to 0.35% for class A shares
   (0.10% distribution fee and 0.25% service fee) and 1.00% for each of class B
   and class C shares (a 0.75% distribution fee and a 0.25% service fee), and
   are paid out of the assets of these classes. Over time, these fees will
   increase the cost of your shares and may cost you more than paying other
   types of sales charges.


                                       12
<PAGE>

---------------------------------------------------
  VI  HOW TO PURCHASE, EXCHANGE AND REDEEM SHARES
---------------------------------------------------

    You may purchase, exchange and redeem class A, B and C shares of the fund
    in the manner described below. In addition, you may be eligible to
    participate in certain investor services and programs to purchase,
    exchange and redeem these classes of shares, which are described in the
    next section under the caption "Investor Services and Programs."

>  How to Purchase Shares

   Initial Purchase. You can establish an account by having your financial
   adviser process your purchase. The minimum initial investment is $1,000.
   However, in the following circumstances the minimum initial investment is
   only $50 per account:

   o if you establish an automatic investment plan;

   o if you establish an automatic exchange plan; or

   o if you establish an account under either:

     > tax-deferred retirement programs (other than IRAs) where investments are
       made by means of group remittal statements; or

     > employer sponsored investment programs.

   The minimum initial investment for IRAs is $250 per account. The maximum
   investment in class C shares is $1,000,000 per transaction. Class C shares
   are not available for purchase by any retirement plan qualified under Section
   401(a) or 403(b) of the Internal Revenue Code if the plan or its sponsor
   subscribes to certain recordkeeping services made available by MFSC, such as
   the MFS Corporate Plan Services Fundamental 401(k) Plan.

   Adding to Your Account. There are several easy ways you can make additional
   investments of at least $50 to your account:

   o send a check with the returnable portion of your statement;

   o ask your financial adviser to purchase shares on your behalf;

   o wire additional investments through your bank (call MFSC first for
     instructions); or

   o authorize transfers by phone between your bank account and your MFS account
     (the maximum purchase amount for this method is $100,000). You must elect
     this privilege on your account application if you wish to use it.


>  How to Exchange Shares

   You can exchange your shares for shares of the same class of certain other
   MFS funds at net asset value by having your financial adviser process your
   exchange request or by contacting MFSC directly. The minimum exchange amount
   is generally $1,000 ($50 for


                                       13
<PAGE>

   exchanges made under the automatic exchange plan). Shares otherwise subject
   to a CDSC will not be charged a CDSC in an exchange. However, when you redeem
   the shares acquired through the exchange, the shares you redeem may be
   subject to a CDSC, depending upon when you originally purchased the shares
   you exchanged. For purposes of computing the CDSC, the length of time you
   have owned your shares will be measured from the date of original purchase
   and will not be affected by any exchange.

   Sales charges may apply to exchanges made from the MFS money market funds.
   Certain qualified retirement plans may make exchanges between the MFS funds
   and the MFS Fixed Fund, a bank collective investment fund, and sales charges
   may also apply to these exchanges. Call MFSC for information concerning these
   sales charges.

   Exchanges may be subject to certain limitations and are subject to the MFS
   funds' policies concerning excessive trading practices, which are policies
   designed to protect the funds and their shareholders from the harmful effect
   of frequent exchanges. These limitations and policies are described below
   under the captions "Right to Reject or Restrict Purchase and Exchange Orders"
   and "Excessive Trading Practices." You should read the prospectus of the MFS
   fund into which you are exchanging and consider the differences in
   objectives, policies and rules before making any exchange.


>  How to Redeem Shares

   You may redeem your shares either by having your financial adviser process
   your redemption or by contacting MFSC directly. The fund sends out your
   redemption proceeds within seven days after your request is received in good
   order. "Good order" generally means that the stock power, written request for
   redemption, letter of instruction or certificate must be endorsed by the
   record owner(s) exactly as the shares are registered. In addition, you need
   to have your signature guaranteed and/or submit additional documentation to
   redeem your shares. See "Signature Guarantee/Additional Documentation" below,
   or contact MFSC for details (see back cover page for address and phone
   number).

   Under unusual circumstances such as when the New York Stock Exchange is
   closed, trading on the Exchange is restricted or if there is an emergency,
   the fund may suspend redemptions or postpone payment. If you purchased the
   shares you are redeeming by check, the fund may delay the payment of the
   redemption proceeds until the check has cleared, which may take up to 15 days
   from the purchase date.

   Redeeming Directly through MFSC.

   o By telephone. You can call MFSC to have shares redeemed from your account
     and the proceeds wired or mailed (depending on the amount redeemed)
     directly to a pre-designated bank account. MFSC will request personal or
     other information from you and will generally record the calls. MFSC will
     be responsible for losses that result from unauthorized telephone
     transactions if it does not follow reasonable procedures designed to verify
     your identity. You must elect this privilege on your account application if
     you wish to use it.


                                       14
<PAGE>

   o By mail. To redeem shares by mail, you can send a letter to MFSC with the
     name of your fund, your account number, and the number of shares or dollar
     amount to be sold.

   Redeeming through Your Financial Adviser. You can call your financial adviser
   to process a redemption on your behalf. Your financial adviser will be
   responsible for furnishing all necessary documents to MFSC and may charge you
   for this service.

   Signature Guarantee/Additional Documentation. In order to protect against
   fraud, the fund requires that your signature be guaranteed in order to redeem
   your shares. Your signature may be guaranteed by an eligible bank, broker,
   dealer, credit union, national securities exchange, registered securities
   association, clearing agency, or savings association. MFSC may require
   additional documentation for certain types of registrations and transactions.
   Signature guarantees and this additional documentation shall be accepted in
   accordance with policies established by MFSC, and MFSC may make certain de
   minimis exceptions to these requirements.


>  Other Considerations

   Right to Reject or Restrict Purchase and Exchange Orders. Purchases and
   exchanges should be made for investment purposes only. The MFS funds each
   reserve the right to reject or restrict any specific purchase or exchange
   request. Because an exchange request involves both a request to redeem shares
   of one fund and to purchase shares of another fund, the MFS funds consider
   the underlying redemption and purchase requests conditioned upon the
   acceptance of each of these underlying requests. Therefore, in the event that
   the MFS funds reject an exchange request, neither the redemption nor the
   purchase side of the exchange will be processed. When a fund determines that
   the level of exchanges on any day may be harmful to its remaining
   shareholders, the fund may delay the payment of exchange proceeds for up to
   seven days to permit cash to be raised through the orderly liquidation of its
   portfolio securities to pay the redemption proceeds. In this case, the
   purchase side of the exchange will be delayed until the exchange proceeds are
   paid by the redeeming fund.

   Excessive Trading Practices. The MFS funds do not permit market-timing or
   other excessive trading practices. Excessive, short-term (market-timing)
   trading practices may disrupt portfolio management strategies and harm fund
   performance. As noted above, the MFS funds reserve the right to reject or
   restrict any purchase order (including exchanges) from any investor. To
   minimize harm to the MFS funds and their shareholders, the MFS funds will
   exercise these rights if an investor has a history of excessive trading or if
   an investor's trading, in the judgment of the MFS funds, has been or may be
   disruptive to a fund. In making this judgment, the MFS funds may consider
   trading done in multiple accounts under common ownership or control.

   Reinstatement Privilege. After you have redeemed shares, you have a one-time
   right to reinvest the proceeds within 90 days of the redemption at the
   current net asset value (without an initial sales charge). If the redemption
   involved a CDSC, your account will be credited with the appropriate amount of
   the CDSC paid; however, your new shares will


                                       15
<PAGE>

   be subject to a CDSC which will be determined from the date you originally
   purchased the shares redeemed.

   In-kind Distributions. The MFS funds have reserved the right to pay
   redemption proceeds by a distribution in-kind of portfolio securities (rather
   than cash). In the event that the fund makes an in-kind distribution, you
   could incur the brokerage and transaction charges when converting the
   securities to cash. The fund does not expect to make in-kind distributions,
   and if it does, the fund will pay, during any 90-day period, your redemption
   proceeds in cash up to either $250,000 or 1% of the fund's net assets,
   whichever is less.

   Involuntary Redemptions/Small Accounts. Because it is costly to maintain
   small accounts, the MFS funds have generally reserved the right to
   automatically redeem shares and close your account when it contains less than
   $500 due to your redemptions or exchanges. Before making this automatic
   redemption, you will be notified and given 60 days to make additional
   investments to avoid having your shares redeemed.


                                       16
<PAGE>

---------------------------------------
  VII  INVESTOR SERVICES AND PROGRAMS
---------------------------------------

   As a shareholder of the fund, you have available to you a number of services
   and investment programs. Some of these services and programs may not be
   available to you if your shares are held in the name of your financial
   adviser or if your investment in the fund is made through a retirement plan.


>  Distribution Options

   The following distribution options are generally available to all accounts
   and you may change your distribution option as often as you desire by
   notifying MFSC:

   o Dividend and capital gain distributions reinvested in additional shares
     (this option will be assigned if no other option is specified);

   o Dividend distributions in cash; capital gain distributions reinvested in
     additional shares; or

   o Dividend and capital gain distributions in cash.

   Reinvestments (net of any tax withholding) will be made in additional full
   and fractional shares of the same class of shares at the net asset value as
   of the close of business on the record date. Distributions in amounts less
   than $10 will automatically be reinvested in additional shares of the fund.
   If you have elected to receive distributions in cash, and the postal or other
   delivery service is unable to deliver checks to your address of record, or
   you do not respond to mailings from MFSC with regard to uncashed distribution
   checks, your distribution option will automatically be converted to having
   all distributions reinvested in additional shares. Your request to change a
   distribution option must be received by MFSC by the record date for a
   distribution in order to be effective for that distribution. No interest will
   accrue on amounts represented by uncashed distribution or redemption checks.


>  Purchase and Redemption Programs

   For your convenience, the following purchase and redemption programs are made
   available to you with respect to class A, B and C shares, without extra
   charge:

   Automatic Investment Plan. You can make cash investments of $50 or more
   through your checking account or savings account on any day of the month. If
   you do not specify a date, the investment will automatically occur on the
   first business day of the month.

   Automatic Exchange Plan. If you have an account balance of at least $5,000 in
   any MFS fund, you may participate in the automatic exchange plan, a
   dollar-cost averaging program. This plan permits you to make automatic
   monthly or quarterly exchanges from your account in an MFS fund for shares of
   the same class of shares of other MFS funds. You may make exchanges of at
   least $50 to up to six different funds under this plan.


                                       17
<PAGE>

   Exchanges will generally be made at net asset value without any sales
   charges. If you exchange shares out of the MFS Money Market Fund or MFS
   Government Money Market Fund, or if you exchange class A shares out of the
   MFS Cash Reserve Fund, into class A shares of any other MFS fund, you will
   pay the initial sales charge if you have not already paid this charge on
   these shares.

   Reinvest Without a Sales Charge. You can reinvest dividend and capital gain
   distributions into your account without a sales charge to add to your
   investment easily and automatically.

   Distribution Investment Program. You may purchase shares of any MFS fund
   without paying an initial sales charge or a CDSC upon redemption by
   automatically reinvesting a minimum of $50 of dividend and capital gain
   distributions from the same class of another MFS fund.

   Letter of Intent (LOI). If you intend to invest $50,000 or more in the MFS
   funds (including the MFS Fixed Fund) within 13 months, you may buy class A
   shares of the funds at the reduced sales charge as though the total amount
   were invested in class A shares in one lump sum. If you intend to invest $1
   million or more under this program, the time period is extended to 36 months.
   If the intended purchases are not completed within the time period, shares
   will automatically be redeemed from a special escrow account established with
   a portion of your investment at the time of purchase to cover the higher
   sales charge you would have paid had you not purchased your shares through
   this program.

   Right of Accumulation. You will qualify for a lower sales charge on your
   purchases of class A shares when your new investment in class A shares,
   together with the current (offering price) value of all your holdings in the
   MFS funds (including the MFS Fixed Fund), reaches a reduced sales charge
   level.

   Systematic Withdrawal Plan. You may elect to automatically receive (or
   designate someone else to receive) regular periodic payments of at least
   $100. Each payment under this systematic withdrawal is funded through the
   redemption of your fund shares. For class B and C shares, you can receive up
   to 10% (15% for certain IRA distributions) of the value of your account
   through these payments in any one year (measured at the time you establish
   this plan). You will incur no CDSC on class B and C shares redeemed under
   this plan. For class A shares, there is no similar percentage limitation;
   however, you may incur the CDSC (if applicable) when class A shares are
   redeemed under this plan.


                                       18
<PAGE>

---------------------------
  VIII  OTHER INFORMATION
---------------------------


>  Pricing of Fund Shares

   The price of each class of the fund's shares is based on its net asset value.
   The net asset value of each class of shares is determined at the close of
   regular trading each day that the New York Stock Exchange is open for trading
   (generally, 4:00 p.m., Eastern time) (referred to as the valuation time). The
   New York Stock Exchange is closed on most national holidays and Good Friday.
   To determine net asset value, the fund values its assets at current market
   values, or at fair value as determined by the adviser under the direction of
   the Board of Trustees that oversees the fund if current market values are
   unavailable. Fair value pricing may be used by the fund when current market
   values are unavailable or when an event occurs after the close of the
   exchange on which the fund's portfolio securities are principally traded that
   is likely to have changed the value of the securities. The use of fair value
   pricing by the fund may cause the net asset value of its shares to differ
   significantly from the net asset value that would be calculated using current
   market values.

   You will receive the net asset value next calculated, after the deduction of
   applicable sales charges and any required tax withholding, if your order is
   complete (has all required information) and MFSC receives your order by:

   o the valuation time, if placed directly by you (not through a financial
     adviser such as a broker or bank) to MFSC; or

   o MFSC's close of business, if placed through a financial adviser, so long as
     the financial adviser (or its authorized designee) received your order by
     the valuation time.

   The fund invests in certain securities which are primarily listed on foreign
   exchanges that trade on weekends and other days when the fund does not price
   its shares. Therefore, the value of the fund's shares may change on days when
   you will not be able to purchase or redeem the fund's shares.


>  Distributions

   The fund intends to pay substantially all of its net income (including any
   realized net capital gains) to shareholders as dividends at least annually.


>  Tax Considerations

   The following discussion is very general. You are urged to consult your tax
   adviser regarding the effect that an investment in the fund may have on your
   particular tax situation.

   Taxability of Distributions. As long as the fund qualifies for treatment as a
   regulated investment company (which it intends to do in its first and each
   subsequent taxable year), it pays no federal income tax on the earnings it
   distributes to shareholders.


                                       19
<PAGE>

   You will normally have to pay federal income taxes, and any state or local
   taxes, on the distributions you receive from the fund, whether you take the
   distributions in cash or reinvest them in additional shares. Distributions
   designated as capital gain dividends are taxable as long-term capital gains.
   Other distributions are generally taxable as ordinary income. Some dividends
   paid in January may be taxable as if they had been paid the previous
   December.

   The Form 1099 that is mailed to you every January details your distributions
   and how they are treated for federal tax purposes.

   Fund distributions will reduce the fund's net asset value per share.
   Therefore, if you buy shares shortly before the record date of a
   distribution, you may pay the full price for the shares and then effectively
   receive a portion of the purchase price back as a taxable distribution.

   If you are neither a citizen nor a resident of the U.S., the fund will
   withhold U.S. federal income tax at the rate of 30% on taxable dividends and
   other payments that are subject to such withholding. You may be able to
   arrange for a lower withholding rate under an applicable tax treaty if you
   supply the appropriate documentation required by the fund. The fund is also
   required in certain circumstances to apply backup withholding at the rate of
   31% on taxable dividends and redemption proceeds paid to any shareholder
   (including a shareholder who is neither a citizen nor a resident of the U.S.)
   who does not furnish to the fund certain information and certifications or
   who is otherwise subject to backup withholding. Backup withholding will not,
   however, be applied to payments that have been subject to 30% withholding.
   Prospective investors should read the fund's Account Application for
   additional information regarding backup withholding of federal income tax.

   Taxability of Transactions. When you redeem, sell or exchange shares, it is
   generally considered a taxable event for you. Depending on the purchase price
   and the sale price of the shares you redeem, sell or exchange, you may have a
   gain or a loss on the transaction. You are responsible for any tax
   liabilities generated by your transaction.


>  Unique Nature of Fund

   MFS may serve as the investment adviser to other funds which have investment
   goals and principal investment policies and risks similar to those of the
   fund, and which may be managed by the fund's portfolio manager(s). While the
   fund may have many similarities to these other funds, its investment
   performance will differ from their investment performance. This is due to a
   number of differences between the funds, including differences in sales
   charges, expense ratios and cash flows.


                                       20
<PAGE>


>  Provision of Annual and Semiannual Reports and Prospectuses

   The fund produces financial reports every six months and updates its
   prospectus annually. To avoid sending duplicate copies of materials to
   households, only one copy of the fund's annual and semiannual report and
   prospectus will be mailed to shareholders having the same residential address
   on the fund's records. However, any shareholder may contact MFSC (see back
   cover for address and phone number) to request that copies of these reports
   and prospectuses be sent personally to that shareholder.


                                       21
<PAGE>

-----------------------
  A p p e n d i x   A
-----------------------


>  Investment Techniques and Practices

   In pursuing its investment objective, the fund may engage in the following
   principal and non-principal investment techniques and practices. Investment
   techniques and practices which are the principal focus of the fund are also
   described, together with their risks, in the Risk Return Summary of the
   Prospectus. Both principal and non-principal investment techniques and
   practices are described, together with their risks, in the SAI.


   Investment Techniques/Practices
   .............................................................................

<TABLE>
<CAPTION>
     Symbols                X permitted                   -- not permitted
     ---------------------------------------------------------------------------
<S>                                                        <C>
     Debt Securities
     Asset-Backed Securities
         Collateralized Mortgage Obligations and
           MulticlassPass-Through Securities               --
         Corporate Asset-Backed Securities                 --
         Mortgage Pass-Through Securities                  --
         Stripped Mortgage-Backed Securities               --
     Corporate Securities                                   X
     Loans and Other Direct Indebtedness                   --
     Lower Rated Bonds                                      X
     Municipal Bonds                                       --
     Speculative Bonds                                      X
     U.S. Government Securities                             X
     Variable and Floating Rate Obligations                 X
     Zero Coupon Bonds, Deferred Interest Bonds and
       PIK Bonds                                            X
     Equity Securities                                      X
     Foreign Securities Exposure
         Brady Bonds                                       --
         Depositary Receipts                                X
         Dollar-Denominated Foreign Debt Securities         X
         Emerging Markets                                   X
         Foreign Securities                                 X
     Forward Contracts                                      X
     Futures Contracts                                      X
     Indexed Securities                                     X
</TABLE>

                                      A-1
<PAGE>


<TABLE>
<S>                                                      <C>
     Inverse Floating Rate Obligations                     --
     Investment in Other Investment Companies
         Open-End Funds                                     X
         Closed-End Funds                                   X
     Lending of Portfolio Securities                        X
     Leveraging Transactions
         Bank Borrowings                                    X
         Mortgage "Dollar-Roll" Transactions               --
         Reverse Repurchase Agreements                     --
     Options
         Options on Foreign Currencies                      X
         Options on Futures Contracts                       X
         Options on Securities                              X
         Options on Stock Indices                           X
         Reset Options                                      X
         "Yield Curve" Options                              X
     Repurchase Agreements                                  X
     Restricted Securities                                  X
     Short Sales                                            X
     Short Sales Against the Box                            X
     Short Term Instruments                                 X
     Swaps and Related Derivative Instruments               X
     Temporary Borrowings                                   X
     Temporary Defensive Positions                          X
     Warrants                                               X
     "When-issued" Securities                               X
</TABLE>



                                      A-2
<PAGE>

   MFS[RegTM] NEW ENDEAVOR FUND

   If you want more information about the fund, the following documents are
   available free upon request:

   Annual/Semiannual Reports. These reports contain information about the fund's
   actual investments. Annual reports discuss the effect of recent market
   conditions and the fund's investment strategy on the fund's performance
   during its last fiscal year.

   Statement of Additional Information (SAI).The SAI, dated October 1, 2000,
   provides more detailed information about the fund and is incorporated into
   this prospectus by reference.

   You can get free copies of the annual/semiannual reports, the SAI and other
   information about the fund, and make inquiries about the fund, by contacting:

          MFS Service Center, Inc.
          2 Avenue de Lafayette
          Boston, MA 02111-1738
          Telephone: 1-800-225-2606
          Internet: http://www.mfs.com

    Information about the fund (including its prospectus, SAI and shareholder
    reports) can be reviewed and copied at the:

          Public Reference Room
          Securities and Exchange Commission
          Washington, D.C., 20549-0102

    Information on the operation of the Public Reference Room may be obtained
    by calling the Commission at 1-202-942-8090. Reports and other information
    about the fund are available on the EDGAR Databases on the Commission's
    Internet website at http://www.sec.gov, and copies of this information may
    be obtained, upon payment of a duplicating fee, by electronic request at
    the following e-mail address: [email protected] or, by writing the Public
    Reference Section at the above address.


          The fund's Investment Company Act file number is 811-4492








                                                 MND-1 12/99 339M 97/297/397/897
<PAGE>



[MFS
INVESTMENT MANAGEMENT
We invented the mutual fund[RegTM] LOGO]


A series of MFS Series Trust X
500 Boylston Street, Boston, MA 02116
(617) 954-5000



This Statement of Additional Information, as amended or supplemented from time
to time (the "SAI"), sets forth information which may be of interest to
investors but which is not necessarily included in the Fund's Prospectus dated
October 1, 2000. This SAI should be read in conjunction with the Prospectus.
You may obtain a copy of the Fund's Prospectus without charge by contacting MFS
Service Center, Inc. (see back cover of Part II of this SAI for address and
phone number).


--------------------------------
  MFS[RegTM] NEW ENDEAVOR FUND
--------------------------------

  O C T O B E R  1 ,  2 0 0 0

                                                         Statement of Additional
                                                                     Information


This SAI is divided into two Parts--Part I and Part II. Part I contains
information that is particular to the Fund, while Part II contains information
that generally applies to each of the funds in the MFS Family of Funds (the
"MFS Funds"). Each Part of the SAI has a variety of appendices which can be
found at the end of Part I and Part II, respectively.

This SAI is NOT a prospectus and is authorized for distribution to prospective
investors only if preceded or accompanied by a current prospectus.










                                                                MND-13 12/99 600
<PAGE>

Statement of Additional Information


PART I

Part I of this SAI contains information that is particular to the Fund.

---------------------
  TABLE OF CONTENTS
---------------------

<TABLE>
<CAPTION>
                                                                                     Page
<S>    <C>                                                                         <C>
I      Definitions ...............................................................   1

II     Management of the Fund ....................................................   1

       The Fund ..................................................................   1

       Trustees and Officers--Identification and Background ......................   1

       Trustee Compensation ......................................................   1

       Affiliated Service Provider Compensation ..................................   1

III    Sales Charges and Distribution Plan Payments ..............................   1

       Sales Charges .............................................................   1

       Distribution Plan Payments ................................................   1

IV     Portfolio Transactions and Brokerage Commissions ..........................   1

V      Share Ownership ...........................................................   1

VI     Performance Information ...................................................   1

VII    Investment Techniques, Practices, Risks and Restrictions ..................   1

       Investment Techniques, Practices and Risks ................................   1

       Investment Restrictions ...................................................   2

VIII   Tax Considerations ........................................................   3

IX     Independent Auditors and Financial Statements .............................   3

       Appendix A--Trustees and Officers--Identification and Background ..........  A-1

       Appendix B--Trustee Compensation ..........................................  B-1

       Appendix C--Affiliated Service Provider Compensation ......................  C-1

       Appendix D--Sales Charges and Distribution Plan Payments ..................  D-1

       Appendix E--Portfolio Transactions and Brokerage Commissions ..............  E-1

       Appendix F--Share Ownership ...............................................  F-1

       Appendix G--Performance Information .......................................  G-1

</TABLE>
<PAGE>

(I)     DEFINITIONS

        "Fund" - MFS New Endeavor Fund, a diversified series of the Trust.

        "Trust" - MFS Series Trust X, a Massachusetts business Trust, organized
        in 1985. The Trust has changed its name several times during the past
        several years. The Trust was previously known as MFS Government Mortgage
        Fund prior to June 2, 1995, MFS Government Income Plus Fund prior to
        March 1, 1993, MFS Government Income Plus Trust prior to August 3, 1992,
        and MFS Government Securities High Yield Trust prior to October 16,
        1989.

        "MFD" - MF Fund Distributors, Inc., a Delaware corporation.

        "Prospectus" - The Prospectus of the Fund, dated October 1, 2000, as
        amended or supplemented from time to time.

(II)    MANAGEMENT OF THE FUND

        The Fund
        The Fund is a diversified series of the Trust. This means that, with
        respect to 75% of its total assets, the Fund may not (1) purchase more
        than 10% of the outstanding voting securities of any one issuer, or (2)
        purchase securities of any issuer if as a result more than 5% of the
        Fund's total assets would be invested in that issuer's securities. This
        limitation does not apply to obligations of the U.S. Government or its
        agencies or instrumentalities. The Trust is an open-end management
        investment company.

        The Fund and its Adviser and Distributor have adopted a code of ethics
        as required under the Investment Company Act of 1940 ("the 1940 Act").
        Subject to certain conditions and restrictions, this code permits
        personnel subject to the code to invest in securities for their own
        accounts, including securities that may be purchased, held or sold by
        the Fund. Securities transactions by some of these persons may be
        subject to prior approval of the Adviser's Compliance Department.
        Securities transactions of certain personnel are subject to quarterly
        reporting and review requirements. The code is on public file with, and
        is available from, the SEC. See the back cover of the prospectus for
        information on obtaining a copy.

        Trustees and Officers - Identification and Background
        The identification and background of the Trustees and officers of the
        Trust are set forth in Appendix A of this Part I.

        Trustee Compensation
        Compensation paid to the non-interested Trustees and to Trustees who
        are not officers of the Trust, for certain specified periods, is set
        forth in Appendix B of this Part I.

        Affiliated Service Provider Compensation
        Compensation paid by the Fund to its affiliated service providers - to
        MFS, for investment advisory and administrative services, and to MFSC,
        for transfer agency services - for certain specified periods is set
        forth in Appendix C to this Part I.

(III)   SALES CHARGES AND DISTRIBUTION PLAN
        PAYMENTS

        Sales Charges
        Sales charges paid in connection with the purchase and sale of Fund
        shares for certain specified periods are set forth in Appendix D to
        this Part I, together with the Fund's schedule of dealer reallowances.

        Distribution Plan Payments
        Payments made by the Fund under the Distribution Plan for its most
        recent fiscal year end are set forth in Appendix D to this Part I.

(IV)    PORTFOLIO TRANSACTIONS AND BROKERAGE
        COMMISSIONS

        Brokerage commissions paid by the Fund for certain specified periods,
        and information concerning purchases by the Fund of securities issued
        by its regular broker-dealers for its most recent fiscal year, are set
        forth in Appendix E to this Part I.

        Broker-dealers may be willing to furnish statistical, research and
        other factual information or services ("Research") to the Adviser for
        no consideration other than brokerage or underwriting commissions.
        Securities may be bought or sold from time to time through such
        broker-dealers, on behalf of the Fund. The Trustees (together with the
        Trustees of certain other MFS Funds) have directed the Adviser to
        allocate a total of $43,800 of commission business from certain MFS
        Funds (including the Fund) to the Pershing Division of Donaldson Lufkin
        & Jenrette as consideration for the annual renewal of certain
        publications provided by Lipper Analytical Securities Corporation
        (which provides information useful to the Trustees in reviewing the
        relationship between the Fund and the Adviser).

(V)     SHARE OWNERSHIP

        Information concerning the ownership of Fund shares by Trustees and
        officers of the Trust as a group, by investors who control the Fund, if
        any, and by investors who own 5% or more of any class of Fund shares,
        if any, is set forth in Appendix F to this Part I.

(VI)    PERFORMANCE INFORMATION

        Performance information, as quoted by the Fund in sales literature and
        marketing materials, is set forth in Appendix G to this Part I.

(VII)   INVESTMENT TECHNIQUES, PRACTICES, RISKS
        AND RESTRICTIONS

        Investment Techniques, Practices and Risks
        The investment objective and principal investment policies of the Fund
        are described in the Prospectus. In pursuing its investment objective
        and principal investment policies, the


                                  Part I -- 1
<PAGE>

        Fund may engage in a number of investment techniques and practices,
        which involve certain risks. These investment techniques and practices,
        which may be changed without shareholder approval unless indicated
        otherwise, are identified in Appendix A to the Prospectus, and are more
        fully described, together with their associated risks, in Part II of
        this SAI. The following percentage limitations apply to these
        investment techniques and practices:

        o Foreign Securities may be up to (but not including) 20% of net assets


        o Lower Rated Bonds may not exceed 10% of net assets

        o Short Sales - value of underlying securities may not exceed 40% of
          net assets

        o Lending of Portfolio Securities may not exceed 30% of the Fund's net
          assets

        Investment Restrictions
        The Fund has adopted the following restrictions which cannot be changed
        without the approval of the holders of a majority of the Fund's shares
        (which, as used in this SAI, means the lesser of (i) more than 50% of
        the outstanding shares of the Trust or the Fund or class, as
        applicable, or (ii) 67% or more of the outstanding shares of the Trust
        or the Fund or class, as applicable, present at a meeting at which
        holders of more than 50% of the outstanding shares of the Trust or the
        Fund or class, as applicable, are represented in person or by proxy).
        Except with respect to the Fund's policy on borrowing and investing in
        illiquid securities, these investment restrictions and policies are
        adhered to at the time of purchase or utilization of assets; a
        subsequent change in circumstances will not be considered to result in
        a violation of policy. In the event of a violation of nonfundamental
        investment policy (1), the Fund will reduce the percentage of its
        assets invested in illiquid investments in due course, taking into
        account the best interests of shareholders.

           Terms used below (such as Options and Futures Contracts) are defined
        in Part II of this SAI.

        The Fund may not:

        (1) borrow amounts in excess of 331/3% of its assets including amounts
            borrowed;

        (2) underwrite securities issued by other persons except insofar as the
            Fund may technically be deemed an underwriter under the Securities
            Act of 1933 in selling a portfolio security;

        (3) purchase or sell real estate (including limited partnership
            interests but excluding securities secured by real estate or
            interests therein and securities of companies, such as real estate
            investment trusts, which deal in real estate or interests therein),
            interests in oil, gas or mineral leases, commodities or commodity
            contracts (excluding Options, Options on Futures Contracts, Options
            on Stock Indices, Options on Foreign Currency and any other type of
            option, Futures Contracts, any other type of futures contract, and
            Forward Contracts) in the ordinary course of its business. The Fund
            reserves the freedom of action to hold and to sell real estate,
            mineral leases, commodities or commodity contracts (including
            Options, Options on Futures Contracts, Options on Stock Indices,
            Options on Foreign Currency and any other type of option, Futures
            Contracts, any other type of futures contract, and Forward
            Contracts) acquired as a result of the ownership of securities;

        (4) issue any senior securities except as permitted by the Investment
            Company Act of 1940, as amended (the "1940 Act"). For purposes of
            this restriction, collateral arrangements with respect to any type
            of option (including Options on Futures Contracts, Options, Options
            on Stock Indices and Options on Foreign Currencies), short sale,
            Forward Contracts, Futures Contracts, any other type of futures
            contract, and collateral arrangements with respect to initial and
            variation margin, are not deemed to be the issuance of a senior
            security;

        (5) make loans to other persons. For these purposes, the purchase of
            short-term commercial paper, the purchase of a portion or all of an
            issue of debt securities, the lending of portfolio securities, or
            the investment of the Fund's assets in repurchase agreements shall
            not be considered the making of a loan; or

        (6) purchase any securities of an issuer of a particular industry, if as
            a result, more than 25% of its gross assets would be invested in
            securities of issuers whose principal business activities are in the
            same industry (except obligations issued or guaranteed by the U.S.
            Government or its agencies and instrumentalities and repurchase
            agreements collateralized by such obligations).

        In addition, the Fund has the following nonfundamental policies which
        may be changed without shareholder approval. The Fund will not:

        (1) invest in illiquid investments, including securities subject to
            legal or contractual restrictions on resale or for which there is no
            readily available market (e.g., trading in the security is
            suspended, or, in the case of unlisted securities, where no market
            exists) if more than 15% of the Fund's net assets (taken at market
            value) would be invested in such securities. Repurchase agreements
            maturing in more than seven days will be deemed to be illiquid for
            purposes of the Fund's limitation on investment in illiquid
            securities. Securities that are not registered under the 1933 Act
            and sold in reliance on Rule 144A thereunder, but are determined to
            be liquid by the Trust's Board of Trustees (or its delegee), will
            not be subject to this 15% limitation;

        (2) invest for the purpose of exercising control or management;


                                  Part I -- 2
<PAGE>

        (3) pledge, mortgage or hypothecate in excess of 331/3% of its gross
            assets. For purposes of this restriction, collateral arrangements
            with respect to any type of option (including Options on Futures
            Contracts, Options, Options on Stock Indices and Options on Foreign
            Currencies), any short sale, any type of futures contract (including
            Futures Contracts), Forward Contracts and payments of initial and
            variation margin in connection therewith, are not considered a
            pledge of assets.

(VIII)  TAX CONSIDERATIONS

        For a discussion of tax considerations, see Part II of this SAI.

(IX)    INDEPENDENT AUDITORS AND FINANCIAL
        STATEMENTS

        Ernst & Young LLP are the Fund's independent auditors, providing audit
        services, tax services, and assistance and consultation with respect to
        the preparation of filings with the Securities and Exchange Commission.


                                  Part I -- 3
<PAGE>

-----------------------
  PART I - APPENDIX A
-----------------------

      TRUSTEES AND OFFICERS - IDENTIFICATION
      AND BACKGROUND

      The Trustees and officers of the Trust are listed below, together with
      their principal occupations during the past five years. (Their titles may
      have varied during that period.)

      Trustees
      J. ATWOOD IVES (born 5/1/36)
      Eastern Enterprises (diversified company), Chairman, Trustee and Chief
      Executive Officer
      Address: Weston, Massachusetts

      LAWRENCE T. PERERA (born 6/23/35)
      Hemenway & Barnes (attorneys), Partner
      Address: Boston, Massachusetts

      WILLIAM J. POORVU (born 4/10/35)
      Harvard University Graduate School of Business Administration, Adjunct
      Professor; CBL & Associates Properties, Inc. (real estate investment
      trust), Director; The Baupost Fund (a mutual fund), Vice Chairman and
      Trustee
      Address: Harvard Business School, Cambridge,
      Massachusetts

      CHARLES W. SCHMIDT (born 3/18/28)
      Private investor; IT Group, Inc. (diversified environmental services and
      consulting), Director
      Address: Weston, Massachusetts

      ARNOLD D. SCOTT* (born 12/16/42)
      Massachusetts Financial Services Company, Senior Executive Vice President
      and Director

      JEFFREY L. SHAMES* Chairman and President (born 6/2/55)
      Massachusetts Financial Services Company, Chairman and Chief Executive
      Officer

      ELAINE R. SMITH (born 4/25/46)
      Independent Consultant
      Address: Weston, Massachusetts

      DAVID B. STONE (born 9/2/27)
      North American Management Corp. (investment adviser), Chairman; Eastern
      Enterprises (diversified services company), Trustee
      Address: Boston, Massachusetts


      Officers
      STEPHEN E. CAVAN,* Clerk and Secretary (born 11/6/53) Massachusetts
      Financial Services Company, Senior Vice President, General Counsel and
      Assistant Secretary

      JAMES R. BORDEWICK, JR.,* Assistant Clerk and Assistant Secretary (born
      3/6/59)
      Massachusetts Financial Services Company, Senior Vice President and
      Associate General Counsel

      JAMES O. YOST,* Treasurer (born 6/12/60) Massachusetts Financial Services
      Company, Senior Vice President

      MARK E. BRADLEY,* Assistant Treasurer (born 11/23/59) Massachusetts
      Financial Services Company, Vice President (since March 1997); Putnam
      Investments, Vice President (from September 1994 until March 1997)

      ELLEN MOYNIHAN,* Assistant Treasurer (born 11/13/57) Massachusetts
      Financial Services Company, Vice President (since September 1996);
      Deloitte & Touche LLP, Senior Manager (prior to September 1996)

      LAURA F. HEALY,* Assistant Treasurer (born 3/20/64) Massachusetts
      Financial Services Company, Vice President (since December 1996); State
      Street Bank Fund Administration Group, Assistant Vice President (prior to
      December 1996)

      ROBERT R. FLAHERTY,* Assistant Treasurer (born 9/18/63)
      Massachusetts Financial Services Company, Vice President (since August
      2000); VAM Fund Services, Senior Vice President (since 1996); Chase
      Global Fund Services, Vice President (1995 to 1996)

      ------------------------------
      *  "Interested persons" (as defined in the Investment Company Act of 1940,
         as amended (the "1940 Act")) of the Adviser, whose address is 500
         Boylston Street, Boston, Massachusetts 02116.


      Each Trustee and officer holds comparable positions with certain MFS
      affiliates or with certain other funds of which MFS or a subsidiary of
      MFS is the investment adviser or distributor. Messrs. Shames and Scott,
      Directors of MFD, and Mr. Cavan, the Secretary of MFD, hold similar
      positions with certain other MFS affiliates.


                                 Part I -- A-1
<PAGE>

-----------------------
  PART I - APPENDIX B
-----------------------


      TRUSTEE COMPENSATION

      The Fund pays the compensation of non-interested Trustees and of Trustees
      who are not officers of the Trust, who currently receive a fee of $250
      per year plus $25 per meeting and $20 per committee meeting attended,
      together with such Trustee's out-of-pocket expenses.

      Trustee Compensation Table
      ..........................................................................

      <TABLE>
      <CAPTION>
                                                     Total Trustee
                                 Trustee Fees       Fees from Fund
      Trustee                    from Fund(1)     and Fund Complex(2)
      ---------------------------------------------------------------
      <S>                             <C>              <C>
      J. Atwood Ives                  $0               $132,623
      Lawrence T. Perera               0                144,098
      William J. Poorvu                0                141,338
      Charles W. Schmidt               0                137,678
      Arnold D. Scott                  0                      0
      Jeffrey L. Shames                0                      0
      Elaine R. Smith                  0                144,098
      David B. Stone                   0                151,418
      </TABLE>

    --------------------
   (1) These fees are estimated for the current fiscal year. The Trustees are
       currently waiving their right to receive fees.
   (2) Information provided is provided for calendar year 1999. All Trustees
       served as Trustees of 34 funds within the MFS fund complex (having
       aggregate net assets at December 31, 1999, of approximately $59 billion).


                                 Part I -- B-1
<PAGE>

------------------------
  PART I -- APPENDIX C
------------------------


AFFILIATED SERVICE PROVIDER COMPENSATION
 ..........................................................................
The Fund paid compensation to its affiliated service providers over the
specified periods as follows:



<TABLE>
<CAPTION>
                        Paid to MFS     Amount     Paid to MFS for       Paid to MFSC       Amount        Aggregate
                       for Advisory     Waived      Administrative       for Transfer       Waived       Amount Paid
 Fiscal Year Ended       Services       by MFS         Services        Agency Services     by MFSC     to MFS and MFSC
----------------------------------------------------------------------------------------------------------------------
<S>                   <C>              <C>        <C>                 <C>                 <C>         <C>
Not Applicable
</TABLE>

-------------------


                                 Part I -- C-1
<PAGE>

-----------------------
  PART I - APPENDIX D
-----------------------


      SALES CHARGES AND DISTRIBUTION PLAN PAYMENTS


      Sales Charges
      ..........................................................................

      The following sales charges were paid during the specified periods:


     <TABLE>
     <CAPTION>
                             Class A Initial Sales Charges:            CDSC Paid to MFD on:
                                      Retained      Reallowed     Class A     Class B     Class C
      Fiscal Year Ended     Total      by MFD      to Dealers      Shares      Shares     Shares
      --------------------------------------------------------   --------------------------------
      <S>                   <C>        <C>          <C>            <C>         <C>         <C>
      Not Applicable
      </TABLE>

      Dealer Reallowances
      ..........................................................................

      As shown above, MFD pays (or "reallows") a portion of the Class A initial
      sales charge to dealers. The dealer reallowance as expressed as a
      percentage of the Class A shares' offering price is:


      <TABLE>
      <CAPTION>
                                                Dealer Reallowance as a
      Amount of Purchase                       percent of Offering Price
      <S>                                                <C>
      Less than $50,000                                  5.00%
      $50,000 but less than $100,000                     4.00%
      $100,000 but less than $250,000                    3.20%
      $250,000 but less than $500,000                    2.25%
      $500,000 but less than $1,000,00  0                1.70%
      $1,000,000 or more                                 None*
      </TABLE>

      --------------
      *  A CDSC will apply to such purchase.


      Distribution Plan Payments
      ..........................................................................

      The Fund is newly organized and has not made payments under the
      Distribution Plan as of the date of this SAI.

      Distribution plan payments retained by MFD are used to compensate MFD for
      commissions advanced by MFD to dealers upon sale of fund shares.


                                 Part I -- D-1
<PAGE>

-----------------------
  PART I - APPENDIX E
-----------------------


      PORTFOLIO TRANSACTIONS AND BROKERAGE COMMISSIONS


      Brokerage Commissions
      ..........................................................................

      The Fund is newly organized and has not paid brokerage commissions as of
      the date of this SAI.


      Securities Issued by Regular Broker-Dealers
      ..........................................................................

      The Fund is newly organized and has not purchased securities issued by its
      regular broker-dealers as of the date of this SAI.


                                 Part I -- E-1
<PAGE>

-----------------------
  PART I - APPENDIX F
-----------------------


      SHARE OWNERSHIP


      Ownership by Trustees and Officers
      Not Applicable.


      25% or Greater Ownership
      The following table identifies those investors who own 25% or more of the
      Fund's shares (all share classes taken together) and are therefore
      presumed to control the Fund:



      <TABLE>
      <CAPTION>
                                      Jurisdiction of Organization
       Name and Address of Investor          (if a Company)         Percentage Ownership
      ----------------------------------------------------------------------------------
              <S>                     <C>                           <C>
               Not Applicable
 </TABLE>

      5% or Greater Ownership of Share Class
      The following table identifies those investors who own 5% or more of any
      class of the Fund's shares:


      <TABLE>
      <CAPTION>
      Name and Address of Investor Ownership                           Percentage
      ---------------------------------------------------------------------------
      <S>                                                                <C>
      Not Applicable
      </TABLE>


                                 Part I -- F-1
<PAGE>

-----------------------
  PART I - APPENDIX G
-----------------------


      PERFORMANCE INFORMATION
      ..........................................................................

      The Fund is newly organized and has no performance quotations as of the
      date of this SAI.

                                 Part I -- G-1



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