MFS VARIABLE INSURANCE TRUST
497, 1997-05-05
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<PAGE>
 
<TABLE>
<S>                                      <C>
                                         PROSPECTUS
MFS-REGISTERED TRADEMARK- EMERGING       May 1,
GROWTH SERIES                            1997
</TABLE>
 
- --------------------------------------------------------------------------------
 
<TABLE>
<S>                                                         <C>
MFS-Registered Trademark- VARIABLE INSURANCE TRUSTSM
</TABLE>
 
MFS  Variable Insurance Trust (the "Trust") is an open-end management investment
company offering insurance company separate  accounts a selection of  investment
vehicles  for  variable  annuity  and  variable  life  insurance  contracts (the
"Contracts"). Currently the  Trust offers  shares of beneficial  interest of  12
separate  mutual fund series (individually  or collectively hereinafter referred
to as a  "Series" or the  "Series"), one of  which is offered  pursuant to  this
Prospectus:
 
- -- MFS  EMERGING GROWTH  SERIES (the "Emerging  Growth Series"),  which seeks to
   provide long-term growth of capital.
                              -------------------
 
THE EMERGING GROWTH  SERIES IS  INTENDED FOR  INVESTORS WHO  UNDERSTAND AND  ARE
WILLING  TO ACCEPT  THE RISKS ENTAILED  IN SEEKING LONG-TERM  GROWTH OF CAPITAL.
BECAUSE OF ITS INVESTMENT POLICIES PERMITTING INVESTMENT IN FOREIGN  SECURITIES,
INVESTMENTS  IN  THE SERIES  MAY BE  SUBJECT TO  A GREATER  DEGREE OF  RISK THAN
INVESTMENTS IN  OTHER INVESTMENT  COMPANIES WHICH  INVEST ENTIRELY  IN  DOMESTIC
SECURITIES.
                              -------------------
 
  THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
    EXCHANGE  COMMISSION  OR ANY  STATE SECURITIES  COMMISSION NOR  HAS THE
     SECURITIES AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION
       PASSED UPON  THE ACCURACY  OR ADEQUACY  OF THIS  PROSPECTUS.  ANY
             REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
                              -------------------
 
SHARES  OF THE TRUST  ARE AVAILABLE AND  ARE BEING MARKETED  AS A POOLED FUNDING
VEHICLE FOR LIFE INSURANCE COMPANIES WRITING ALL TYPES OF CONTRACTS.
 
This Prospectus sets  forth concisely the  information about the  Series that  a
prospective  investor should know  before applying for  the Contracts offered by
the separate accounts of  certain insurance companies ("Participating  Insurance
Companies").  Investors are advised  to read this  Prospectus and the applicable
Contract prospectus  carefully and  retain  them for  future reference.  If  you
require  more detailed information, a  Statement of Additional Information dated
May 1,  1997, as  amended or  supplemented from  time to  time (the  "SAI"),  is
available  upon request  without charge  and may  be obtained  by calling  or by
writing to the Shareholder Servicing Agent (see back cover for address and phone
number). The SAI, which is incorporated  by reference into this Prospectus,  has
been  filed with  the Securities  and Exchange  Commission (the  "SEC"). The SEC
maintains an Internet World Wide Web site that contains the SAI, materials  that
are  incorporated  by reference  into  this Prospectus  and  the SAI,  and other
information regarding the Series. This Prospectus is available on the  Adviser's
Internet World Wide Web site at http://www.mfs.com.
 
   INVESTORS SHOULD READ THIS PROSPECTUS AND RETAIN IT FOR FUTURE REFERENCE.
<PAGE>
TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                                                                                       PAGE
                                                                                                       ---
  <S>                                                                                                  <C>
  1.  Expense Summary................................................................................   3
  2.  Investment Concept of the Trust................................................................   3
  3.  Condensed Financial Information................................................................   4
  4.  Investment Objectives and Policies.............................................................   6
      MFS Emerging Growth Series.....................................................................   6
  5.  Investment Techniques..........................................................................   6
  6.  Additional Risk Factors........................................................................  10
  7.  Management of the Series.......................................................................  13
  8.  Information Concerning Shares of the Series....................................................  14
      Purchases and Redemptions......................................................................  14
      Net Asset Value................................................................................  15
      Distributions..................................................................................  15
      Tax Status.....................................................................................  15
      Description of Shares, Voting Rights and Liabilities...........................................  15
      Performance Information........................................................................  16
      Expenses.......................................................................................  16
      Shareholder Communications.....................................................................  17
  Appendix A -- Description of Bond Ratings..........................................................  A-1
</TABLE>
 
                                       2
<PAGE>
1.  EXPENSE SUMMARY
 
ANNUAL OPERATING EXPENSES (AS A PERCENTAGE OF AVERAGE NET ASSETS):
 
<TABLE>
<CAPTION>
                                                                MFS
                                                              EMERGING
                                                               GROWTH
                                                               SERIES
                                                              --------
<S>                                                           <C>
Management Fee..............................................  0.75%
Other Expenses (after expense limitation)(1)(2).............  0.25%
                                                              --------
Total Operating Expenses (after expense limitation)(2)......  1.00%
</TABLE>
 
- ------------------------
 
<TABLE>
<S>        <C>
(1)        The  Series has an expense  offset arrangement which reduces  the Series' custodian fee  based upon the amount of
           cash maintained by the Series  with its custodian and  dividend disbursing agent, and  may enter into other  such
           arrangements  and directed  brokerage arrangements  (which would  also have  the effect  of reducing  the Series'
           expenses). Any such fee reductions are not reflected under "Other Expenses."
(2)        The Adviser has agreed to bear expenses for the Series, subject to reimbursement by the Series, such that the
           Series' "Other Expenses" shall not exceed 0.25% of the average daily net assets of the Series during the current
           fiscal year. See "Information Concerning Shares of the Series--Expenses." Otherwise, "Other Expenses" and "Total
           Operating Expenses" for the Series would be 0.41% and 1.16%, respectively.
</TABLE>
 
    The  purpose  of  the  expense  table  above  is  to  assist  investors   in
understanding  the various costs  and expenses that a  shareholder of the Series
will bear directly or indirectly. The  Series' annual operating expenses do  not
reflect  expenses  imposed  by  separate  accounts  of  Participating  Insurance
Companies through which  an investment in  the Series is  made or their  related
Contracts. A separate account's expenses are disclosed in the prospectus through
which the Contract relating to that separate account is offered for sale.
 
2.  INVESTMENT CONCEPT OF THE TRUST
 
    The Trust is an open-end, registered management investment company comprised
of  the following twelve series: Emerging  Growth Series, Value Series, Research
Series, Growth With Income Series,  Total Return Series, Utilities Series,  High
Income  Series, World  Governments Series,  Strategic Fixed  Income Series, Bond
Series, Limited  Maturity Series  and  Money Market  Series.  Each Series  is  a
segregated,  separately  managed portfolio  of  securities. All  of  the Series,
except the Utilities Series, World Governments Series and Strategic Fixed Income
Series, are diversified. Additional series may be created from time to time. The
Trust was organized as a  business trust under the  laws of The Commonwealth  of
Massachusetts by a Declaration of Trust dated February 1, 1994.
 
    The  Trust  currently  offers shares  of  each Series  to  insurance company
separate accounts that fund Contracts. Separate accounts may purchase or  redeem
shares  at net  asset value  without any  sales or  redemption charge.  Fees and
charges imposed by a separate account, however, will affect the actual return to
the  holder  of  a  Contract.  A  separate  account  may  also  impose   certain
restrictions  or limitations on the allocation  of purchase payments or Contract
value to one or more Series, and  not all Series may be available in  connection
with  a particular Contract. Prospective investors should consult the applicable
Contract prospectus for information regarding fees and expenses of the  Contract
and  separate account and any applicable  restrictions or limitations. The Trust
assumes no responsibility for such prospectuses.
 
    Shares of the Series are offered  to the separate accounts of  Participating
Insurance  Companies  that are  affiliated  or unaffiliated  ("shared funding").
Shares of the Series may serve  as the underlying investments for both  variable
annuity  and  variable  life  insurance  contracts  ("mixed  funding").  Due  to
differences in tax treatment or  other considerations, the interests of  various
Contract  owners might at some time be in conflict. The Trust currently does not
foresee any such conflict. Nevertheless, the Trust's Trustees intend to  monitor
events  in order  to identify  any material  irreconcilable conflicts  which may
possibly arise and to determine what action, if any, should be taken in response
thereto. If such a conflict were to occur, one or more separate accounts of  the
Participating  Insurance Companies might be required to withdraw its investments
in one one  or more  Series. This  might force a  Series to  sell securities  at
disadvantageous prices.
 
    Individual Contract holders are not the "shareholders" of the Trust. Rather,
the  Participating  Insurance  Companies  and their  separate  accounts  are the
shareholders or  investors,  although such  companies  may pass  through  voting
rights to their Contract holders.
 
                                       3
<PAGE>
    The Trust's Board of Trustees provides broad supervision over the affairs of
the  Trust and the Series. Massachusetts  Financial Services Company, a Delaware
corporation ("MFS" or the "Adviser"), is the investment adviser to each  Series.
A majority of the Trustees of the Trust are not affiliated with the Adviser. The
Adviser  is responsible for the management of  the assets of each Series and the
officers of the Trust  are responsible for the  operations. The Adviser  manages
the  Series'  portfolios  from day  to  day  in accordance  with  the investment
objectives and policies of each Series. The selection of investments and the way
they are managed  depend on the  conditions and  trends in the  economy and  the
financial marketplaces.
 
3.  CONDENSED FINANCIAL INFORMATION
 
The  following financial information has been  audited since the commencement of
investment operations of the Series and  should be read in conjunction with  the
financial  statements  included in  the Series'  Annual Report  to Shareholders.
These financial  statements  are  incorporated  by reference  into  the  SAI  in
reliance  upon the report  of the Series' independent  auditors given upon their
authority as experts in accounting and auditing. The Series' current independent
auditors are Deloitte & Touche LLP.
 
                                       4
<PAGE>
                             EMERGING GROWTH SERIES
 
<TABLE>
<CAPTION>
                                                                      YEAR ENDED          PERIOD ENDED
                                                                   DECEMBER 31, 1996   DECEMBER 31, 1995*
                                                                   -----------------   ------------------
<S>                                                                <C>                 <C>
Per share data (for a share outstanding throughout each period):
Net asset value--beginning of period.............................      $  11.41             $10.00
                                                                       --------             ------
Income from investment operations#--
  Net investment income (loss)Section............................      $  (0.01)            $ 0.01
  Net realized and unrealized gain on investments and foreign
    currency transactions........................................          1.95               1.74
                                                                       --------             ------
    Total from investment operations.............................      $   1.94             $ 1.75
                                                                       --------             ------
Less distributions declared to shareholders--
  From net investment income.....................................      $     --             $(0.01)
  From net realized gain on investments and foreign currency
    transactions.................................................         (0.06)             (0.26)
  In excess of net realized gain on investments and foreign
    currency transactions........................................         (0.05)                --
  Tax return of capital..........................................            --              (0.07)
                                                                       --------             ------
    Total distributions declared to shareholders.................      $  (0.11)            $(0.34)
                                                                       --------             ------
Net asset value--end of period...................................      $  13.24             $11.41
                                                                       --------             ------
                                                                       --------             ------
Total return.....................................................         17.02%             17.41%++
Ratios (to average net assets)/Supplemental dataSection:
  Expenses.......................................................          1.00%              1.00%+
  Net investment income (loss)...................................         (0.08)%             0.10%+
Portfolio turnover...............................................            96%                73%
Average commission rate###.......................................      $ 0.0401                 --
Net assets at end of period (000 omitted)........................      $104,956             $3,869
</TABLE>
 
- --------------------------
 
<TABLE>
<C>        <S>
        *  For the period from the commencement of investment operations, July 24, 1995 to December 31, 1995.
        +  Annualized.
       ++  Not annualized.
        #  Per share data is based on average shares outstanding.
      ###  Average commission rate is calculated for Series' with fiscal years beginning on or after September 1, 1995.
  Section  The Adviser voluntarily agreed to maintain the expenses of the Series at not more than 1.00% of average daily
           net assets. To the extent actual expenses were over these limitations, the net investment loss per share  and
           the ratios would have been:
 
Net investment loss..............................................         $(0.03)           $(0.18)
Ratios (to average net assets):
  Expenses.......................................................           1.16%             2.91%+
  Net investment loss............................................          (0.23)%           (1.78)%+
</TABLE>
 
                                       5
<PAGE>
4.  INVESTMENT OBJECTIVES AND POLICIES
 
    The  investment objectives and policies of  the Series may, unless otherwise
specifically stated, be changed by the Trustees  of the Trust without a vote  of
the  shareholders. Any investment  involves risk and there  is no assurance that
the objectives of the Series will be achieved.
 
    In addition to the specific investment practices described below, the Series
may also engage in certain investment techniques as described under the  caption
"Investment  Techniques"  below and  in the  SAI  under the  caption "Investment
Techniques." The Series' investments are subject to certain risks, as  described
in the above-referenced sections of this Prospectus and the SAI and as described
below under the caption "Additional Risk Factors."
 
MFS  EMERGING GROWTH SERIES --  The Series seeks to  provide long-term growth of
capital. Dividend  and interest  income from  portfolio securities,  if any,  is
incidental to the Series' investment objective of long-term growth of capital.
 
    The  Series' policy is to invest primarily (I.E., at least 80% of its assets
under normal circumstances) in common stocks of companies that MFS believes  are
early  in  their  life  cycle  but which  have  the  potential  to  become major
enterprises (emerging  growth  companies).  Such companies  generally  would  be
expected to show earnings growth over time that is well above the growth rate of
the  overall economy  and the  rate of inflation,  and would  have the products,
technologies, management and  market and other  opportunities which are  usually
necessary  to become more widely recognized as growth companies. Emerging growth
companies can  be of  any size,  and the  Series may  invest in  larger or  more
established  companies 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. While
the Series will invest primarily in common stocks, the Series may, to a  limited
extent,  seek appreciation  in other  types of  securities such  as fixed income
securities (which  may be  unrated), convertible  securities and  warrants  when
relative  values  make such  purchases  appear attractive  either  as individual
issues or as types  of securities in certain  economic environments. The  Series
may   invest  in  non-convertible  fixed  income  securities  rated  lower  than
"investment grade"  (rated  Ba  or  lower by  Moody's  Investors  Service,  Inc.
("Moody's")  or BB  or lower  by Standard &  Poor's Ratings  Services ("S&P") or
Fitch Investors Service, Inc. ("Fitch")) (commonly known as "junk bonds") or  in
comparable  unrated securities,  when, in  the opinion  of the  Adviser, such an
investment presents a greater opportunity for appreciation with comparable  risk
to   an  investment  in  "investment  grade"  securities.  Under  normal  market
conditions, the Series will invest not more  than 5% of its net assets in  these
securities.  For  a  description  of  these  ratings,  see  Appendix  A  to this
Prospectus.
 
    The nature of investing in  emerging growth companies involves greater  risk
than  is customarily associated with  investments in more established companies.
Emerging growth companies often have limited product lines, markets or financial
resources, and  they may  be dependent  on one-person  management. In  addition,
there  may be less research  available on many promising  small and medium sized
emerging growth companies, making  it more difficult to  find and analyze  these
companies.  The  securities  of  emerging  growth  companies  may  have  limited
marketability and may be subject to more abrupt or erratic market movements than
securities of larger, more established  growth companies or the market  averages
in  general. Shares of the Series, therefore, are subject to greater fluctuation
in value than shares  of a conservative  equity fund or of  a growth fund  which
invests entirely in proven growth stocks.
 
    Consistent  with its investment objective  and policies described above, the
Series may also invest up to 25% (and generally expects to invest not more  than
15%)  of  its  net  assets  in  foreign  securities  (including  emerging market
securities and Brady Bonds) which are not traded on a U.S. exchange.
 
5.  INVESTMENT TECHNIQUES
 
    LENDING OF PORTFOLIO SECURITIES: The Series may seek to increase its  income
by lending portfolio securities. Such loans will usually be made to member firms
(and  subsidiaries thereof) of the New  York Stock Exchange (the "Exchange") and
to member banks  of the  Federal Reserve  System, and  would be  required to  be
secured  continuously  by collateral  in cash,  U.S.  Treasury securities  or an
irrevocable letter of credit maintained on a current basis at an amount at least
equal to the market value of the securities loaned. If the Adviser determines to
make securities loans, it  is intended that the  value of the securities  loaned
would  not exceed 10%  of the value of  the net assets of  the Series making the
loans.
 
                                       6
<PAGE>
    EMERGING  MARKET SECURITIES: Consistent with  its respective objectives, the
Series may  invest  in securities  of  issuers whose  principal  activities  are
located  in  emerging market  countries. Emerging  market countries  include any
country determined by  the Adviser to  have an emerging  market economy,  taking
into  account a number of  factors, including whether the  country has a low- to
middle- income economy  according to the  International Bank for  Reconstruction
and  Development, the country's foreign currency  debt rating, its political and
economic stability and the development of its financial and capital markets. The
Adviser determines whether an  issuer's principal activities  are located in  an
emerging   market  country  by  considering  such  factors  as  its  country  of
organization, the principal trading market for its securities and the source  of
its  revenues and assets. The issuer's principal activities generally are deemed
to be  located  in a  particular  country if:  (a)  the security  is  issued  or
guaranteed by the government of that country or any of its agencies, authorities
or  instrumentalities;  (b)  the issuer  is  organized  under the  laws  of, and
maintains a principal office in that  country; (c) the issuer has its  principal
securities trading market in that country; (d) the issuer derives 50% or more of
its total revenues from goods sold or services performed in that country; or (e)
the issuer has 50% or more of its assets in that country.
 
    BRADY  BONDS: The  Series may  invest in  Brady Bonds,  which are securities
created through the  exchange of existing  commercial bank loans  to public  and
private  entities in certain  emerging markets for new  bonds in connection with
debt restructurings under a  debt restructuring plan  introduced by former  U.S.
Secretary of the Treasury, Nicholas F. Brady (the "Brady Plan"). Brady Plan debt
restructurings  have been  implemented to  date in  Argentina, Brazil, Bulgaria,
Costa Rica, Dominican  Republic, Ecuador, Jordan,  Mexico, Nigeria, Panama,  the
Philippines,  Poland, Uruguay and  Venezuela. Brady Bonds  have been issued only
recently, and for that reason  do not have a  long payment history. Brady  Bonds
may be collateralized or uncollateralized, are issued in various currencies (but
primarily the U.S. dollar) and are actively traded in over-the-counter secondary
markets.  U.S.  dollar-denominated,  collateralized Brady  Bonds,  which  may be
fixed-rate bonds or floating-rate bonds, are generally collateralized in full as
to principal by U.S. Treasury zero coupon bonds having the same maturity as  the
bonds.  Brady  Bonds  are  often  viewed  as  having  three  or  four  valuation
components: the collateralized  repayment of  principal at  final maturity;  the
collateralized  interest payments;  the uncollateralized  interest payments; and
any uncollateralized repayment of principal at maturity (these  uncollateralized
amounts  constituting the  "residual risk").  In light  of the  residual risk of
Brady Bonds and the  history of defaults of  countries issuing Brady Bonds  with
respect  to commercial bank loans by public and private entities, investments in
Brady Bonds may be viewed as speculative.
 
    REPURCHASE AGREEMENTS: The  Series may enter  into repurchase agreements  in
order  to earn  income on  available cash or  as a  temporary defensive measure.
Under a  repurchase agreement,  the Series  acquires securities  subject to  the
seller's  agreement to repurchase at  a specified time and  price. If the seller
becomes subject to  a proceeding  under the bankruptcy  laws or  its assets  are
otherwise subject to a stay order, the Series' right to liquidate the securities
may be restricted (during which time the value of the securities could decline).
As  discussed in the SAI, the Series  has adopted certain procedures intended to
minimize risk.
 
    "WHEN-ISSUED"  SECURITIES:  The   Series  may  purchase   securities  on   a
"when-issued"  or on a "forward delivery" basis, which means that the securities
will be  delivered to  the Series  at  a future  date usually  beyond  customary
settlement time. The commitment to purchase a security for which payment will be
made  on a future date may be deemed a separate security. In general, the Series
does not pay  for such  securities until received,  and does  not start  earning
interest on the securities until the contractual settlement date. While awaiting
delivery  of securities purchased on such bases, the Series will normally invest
in cash, cash equivalents and high grade debt securities.
 
    RESTRICTED SECURITIES:  The  Series may  purchase  securities that  are  not
registered  under  the  Securities Act  of  1933 (the  "1933  Act") ("restricted
securities"), including  those  that  can  be offered  and  sold  to  "qualified
institutional   buyers"  under  Rule  144A  under   the  1933  Act  ("Rule  144A
securities"). A determination  is made  based upon  a continuing  review of  the
trading  markets for  a specific  Rule 144A  security, whether  such security is
liquid and thus not subject to the Series' limitation on investing not more than
15% of its net assets in illiquid investments. The Board of Trustees has adopted
guidelines and delegated to MFS the daily function of determining and monitoring
the liquidity of Rule  144A securities. The  Board, however, retains  oversight,
focusing   on  factors  such   as  valuation,  liquidity   and  availability  of
information. Investing  in  Rule  144A  securities  could  have  the  effect  of
decreasing  the level of  liquidity in the  Series to the  extent that qualified
institutional buyers  become for  a time  uninterested in  purchasing Rule  144A
securities held in the Series' portfolio.
 
                                       7
<PAGE>
    CORPORATE  ASSET-BACKED  SECURITIES:  The  Series  may  invest  in corporate
asset-backed securities. These securities, issued by trusts and special  purpose
corporations, are backed by a pool of assets, such as credit card and automobile
loan receivables, representing the obligations of a number of different parties.
 
    Corporate  asset-backed securities  present certain risks.  For instance, in
the case of credit card receivables,  these securities may not have the  benefit
of  any security interest in the related collateral. Credit card receivables are
generally unsecured and the debtors are  entitled to the protection of a  number
of  state and federal consumer credit laws,  many of which give such debtors the
right to set off certain amounts owed on the credit cards, thereby reducing  the
balance  due. Most  issuers of  automobile receivables  permit the  servicers to
retain possession of the  underlying obligations. If the  servicer were to  sell
these  obligations to another  party, there is  a risk that  the purchaser would
acquire an interest superior  to that of the  holders of the related  automobile
receivables.  In addition, because of the large number of vehicles involved in a
typical issuance and technical  requirements under state  laws, the trustee  for
the  holders  of  the automobile  receivables  may  not have  a  proper security
interest in all of the obligations backing such receivables. Therefore, there is
the possibility  that recoveries  on  repossessed collateral  may not,  in  some
cases,  be available  to support  payments on  these securities.  The underlying
assets  (E.G.,  loans)  are  also  subject  to  prepayments  which  shorten  the
securities' weighted average life and may lower their return.
 
    Corporate  asset-backed  securities are  often backed  by  a pool  of assets
representing the obligations  of a number  of different parties.  To lessen  the
effect  of  failures by  obligors  on underlying  assets  to make  payments, the
securities  may  contain  elements  of  credit  support  which  fall  into   two
categories:  (i)  liquidity  protection;  and  (ii)  protection  against  losses
resulting from  ultimate  default  by  an  obligor  on  the  underlying  assets.
Liquidity  protection  refers to  the provision  of  advances, generally  by the
entity administering the pool of assets, to ensure that the receipt of  payments
on  the underlying  pool occurs in  a timely fashion.  Protection against losses
resulting from ultimate  default ensures payment  through insurance policies  or
letters  of credit  obtained by  the issuer or  sponsor from  third parties. The
Series will not  pay any  additional or separate  fees for  credit support.  The
degree  of  credit  support  provided  for  each  issue  is  generally  based on
historical information respecting the level  of credit risk associated with  the
underlying  assets. Delinquency or loss in excess of that anticipated or failure
of the credit support could adversely affect the return on an investment in such
a security.
 
    LOAN PARTICIPATIONS AND OTHER DIRECT  INDEBTEDNESS: The Series may invest  a
portion of its assets in "loan participations" and other direct indebtedness. By
purchasing a loan participation, the Series acquires some or all of the interest
of  a bank or other lending institution in  a loan to a corporate borrower. Many
such loans are secured, and most impose restrictive covenants which must be  met
by  the borrower.  These loans  are made  generally to  finance internal growth,
mergers, acquisitions, stock repurchases, leveraged buy-outs and other corporate
activities. Such loans may be in default at the time of purchase. The Series may
also purchase other direct  indebtedness such as trade  or other claims  against
companies,  which generally represent money owed by the company to a supplier of
goods and  services. These  claims may  also be  purchased at  a time  when  the
company  is  in default.  Certain of  the loan  participations and  other direct
indebtedness acquired by the Series  may involve revolving credit facilities  or
other  standby financing commitments  which obligate a  Series to pay additional
cash on a certain date or on demand.
 
    The highly leveraged nature of many such loans and other direct indebtedness
may make such  loans especially  vulnerable to  adverse changes  in economic  or
market  conditions. Loan participations and other direct indebtedness may not be
in the form of  securities or may  be subject to  restrictions on transfer,  and
only  limited opportunities may  exist to resell such  instruments. As a result,
the Series may be unable  to sell such investments at  an opportune time or  may
have  to resell them at less than fair market value. For a further discussion of
loan  participations,  other  direct  indebtedness  and  the  risks  related  to
transactions therein, see the SAI.
 
    OPTIONS  ON SECURITIES:  The Series  may write  (sell) covered  put and call
options and purchase put and call  options on securities. The Series will  write
options on securities for the purpose of increasing its return and/or to protect
the  value of its  portfolio. In particular,  where the Series  writes an option
that expires unexercised or  is closed out  by the Series at  a profit, it  will
retain  the premium paid for the option which will increase its gross income and
will offset in part the reduced  value of the portfolio security underlying  the
option,  or  the  increased cost  of  portfolio  securities to  be  acquired. In
contrast, however, if the  price of the underlying  security moves adversely  to
the  Series'  position, the  option  may be  exercised  and the  Series  will be
required to purchase or sell the underlying security at a disadvantageous price,
which may only be
 
                                       8
<PAGE>
partially  offset  by the  amount  of the  premium.  The Series  may  also write
combinations of put and call options on the same security, known as "straddles."
Such transactions  can  generate  additional premium  income  but  also  present
increased risk.
 
    By writing a call option on a security, the Series limits its opportunity to
profit  from any increase in the market  value of the underlying security, since
the holder will usually exercise  the call option when  the market value of  the
underlying  security exceeds the exercise price of the call. However, the Series
retains the risk of depreciation in value of securities on which it has  written
call options.
 
    The  Series may also purchase put or  call options in anticipation of market
fluctuations which may adversely affect the value of its portfolio or the prices
of securities that the Series  wants to purchase at a  later date. In the  event
that  the expected market fluctuations  occur, the Series may  be able to offset
the resulting adverse effect on its portfolio, in whole or in part, through  the
options  purchased.  The  premium  paid  for  a  put  or  call  option  plus any
transaction costs will reduce the benefit,  if any, realized by the Series  upon
exercise  or liquidation of the option, and,  unless the price of the underlying
security changes  sufficiently,  the option  may  expire without  value  to  the
Series.
 
    In  certain  instances,  the  Series  may  enter  into  options  on Treasury
securities that  are  "reset"  options or  "adjustable  strike"  options.  These
options provide for periodic adjustment of the strike price and may also provide
for  the periodic adjustment of  the premium during the  term of the option. The
SAI contains a further discussion of these investments.
 
    OPTIONS ON STOCK INDICES: The Series  may write (sell) covered call and  put
options and purchase call and put options on stock indices. The Series may write
options  on stock indices for the purpose  of increasing its gross income and to
protect its portfolio  against declines in  the value of  securities it owns  or
increases  in the value of securities to  be acquired. When the Series writes an
option on a  stock index,  and the  value of the  index moves  adversely to  the
holder's  position, the option will not be exercised, and the Series will either
close out the option at a profit  or allow it to expire unexercised. The  Series
will  thereby retain the amount of  the premium, less related transaction costs,
which will increase its  gross income and  offset part of  the reduced value  of
portfolio  securities or the  increased cost of securities  to be acquired. Such
transactions, however, will constitute only partial hedges against adverse price
fluctuations, since any such fluctuations will  be offset only to the extent  of
the  premium received by the Series for  the writing of the option, less related
transaction costs.  In addition,  if  the value  of  an underlying  index  moves
adversely  to the Series' option position, the  option may be exercised, and the
Series will experience a loss which may  only be partially offset by the  amount
of the premium received.
 
    The  Series may also purchase put or call options on stock indices in order,
respectively, to hedge its investments against a decline in value or to  attempt
to  reduce the risk of missing a market or industry segment advance. The Series'
possible loss in either case will be limited to the premium paid for the option,
plus related transaction costs.
 
    FUTURES CONTRACTS AND OPTIONS ON FUTURES CONTRACTS: The Series may  purchase
and  sell Futures  Contracts on  stock indices, and  may also  purchase and sell
Futures Contracts on foreign  currencies or indices  of foreign currencies.  The
Series may also purchase and write Options on such Futures Contracts.
 
    Such  transactions  will  be  entered  into  for  hedging  purposes  or  for
non-hedging purposes to the extent permitted by applicable law. The Series  will
incur  brokerage fees when it purchases and sells Futures Contracts, and will be
required to  maintain margin  deposits. In  addition, Futures  Contracts  entail
risks. Although the Adviser believes that use of such Contracts will benefit the
Series, if its investment judgment about the general direction of exchange rates
or  the stock market is incorrect, the Series' overall performance may be poorer
than if it had not entered into any  such contract and the Series may realize  a
loss.  The  Series  will not  enter  into  any Futures  Contract  if immediately
thereafter the value  of securities  and other obligations  underlying all  such
Futures  Contracts held by the Series would exceed 50% of the value of its total
assets.
 
    Purchases of Options on Futures Contracts  may present less risk in  hedging
the  Series'  portfolio than  the  purchase or  sale  of the  underlying Futures
Contracts since the potential loss is limited to the amount of the premium  plus
related  transaction costs, although it may  be necessary to exercise the option
to realize any profit, which results in the establishment of a futures position.
The writing of Options on Futures Contracts, however, does not present less risk
than the trading of Futures Contracts and will constitute only a partial  hedge,
up  to  the  amount  of the  premium  received.  In addition,  if  an  option is
exercised, the Series may suffer a loss on the transaction.
 
                                       9
<PAGE>
    Futures Contracts and Options on Futures Contracts that are entered into  by
the Series will be traded on U.S. and foreign exchanges.
 
    FORWARD  CONTRACTS:  The  Series  may enter  into  forward  foreign currency
exchange contracts for the  purchase or sale  of a fixed  quantity of a  foreign
currency  at  a future  date ("Forward  Contracts"). The  Series may  enter into
Forward Contracts  for  hedging purposes  and  for non-hedging  purposes  (I.E.,
speculative  purposes). By entering  into transactions in  Forward Contracts for
hedging purposes,  the  Series  may  be  required  to  forego  the  benefits  of
advantageous  changes in  exchange rates and,  in the case  of Forward Contracts
entered into for non-hedging purposes, the Series may sustain losses which  will
reduce  its  gross income.  Such  transactions, therefore,  could  be considered
speculative. Forward Contracts are traded over-the-counter and not on  organized
commodities or securities exchanges. As a result, Forward Contracts operate in a
manner distinct from exchange-traded instruments, and their use involves certain
risks  beyond those associated with transactions in Futures Contracts or options
traded on  exchanges. The  Series may  choose  to, or  be required  to,  receive
delivery  of the foreign currencies underlying  Forward Contracts it has entered
into. Under certain circumstances, such as  where the Adviser believes that  the
applicable  exchange rate is unfavorable at the time the currencies are received
or the Adviser anticipates,  for any other reason,  that the exchange rate  will
improve,  the Series may hold such currencies  for an indefinite period of time.
The Series may  also enter  into a  Forward Contract  on one  currency to  hedge
against risk of loss arising from fluctuations in the value of a second currency
(referred  to  as  a  "cross hedge")  if,  in  the judgment  of  the  Adviser, a
reasonable degree of correlation can be expected between movements in the values
of the two  currencies. The  Series has established  procedures consistent  with
statements  of the SEC and  its staff regarding the  use of Forward Contracts by
registered investment  companies, which  requires use  of segregated  assets  or
"cover" in connection with the purchase and sale of such contracts.
 
    OPTIONS  ON FOREIGN CURRENCIES: The Series may purchase and write options on
foreign  currencies  ("Options  on  Foreign  Currencies")  for  the  purpose  of
protecting  against declines  in the  dollar value  of portfolio  securities and
against increases in the  dollar cost of  securities to be  acquired. As in  the
case  of other types  of options, however,  the writing of  an Option on Foreign
Currency will constitute only a partial hedge,  up to the amount of the  premium
received,  and the Series may be required to purchase or sell foreign currencies
at disadvantageous exchange rates, thereby incurring losses. The purchase of  an
Option   on  Foreign  Currency   may  constitute  an   effective  hedge  against
fluctuations in exchange rates although, in the event of rate movements  adverse
to  the Series' position, it  may forfeit the entire  amount of the premium paid
for the option plus related transaction costs. The Series may also choose to, or
be required to, receive delivery of the foreign currencies underlying Options on
Foreign Currencies it  has entered  into. Under certain  circumstances, such  as
where  the Adviser believes that the  applicable exchange rate is unfavorable at
the time the currencies are received  or the Adviser anticipates, for any  other
reason, that the exchange rate will improve, the Series may hold such currencies
for an indefinite period of time.
 
6.  ADDITIONAL RISK FACTORS
 
    OPTIONS,  FUTURES CONTRACTS AND FORWARD  CONTRACTS: Although the Series will
enter into  transactions  in  options, Futures  Contracts,  Options  on  Futures
Contracts,  Forward  Contracts and  Options  on Foreign  Currencies  for hedging
purposes, such transactions nevertheless involve  certain risks. For example,  a
lack  of  correlation between  the instrument  underlying  an option  or Futures
Contract and the  assets being  hedged, or unexpected  adverse price  movements,
could  render  the Series'  hedging strategy  unsuccessful  and could  result in
losses. The  Series  also  may  enter  into  transactions  in  options,  Futures
Contracts,  Options on  Futures Contracts and  Forward Contracts  for other than
hedging purposes, which involves greater risk. In particular, such  transactions
may  result in  losses for  the Series which  are not  offset by  gains on other
portfolio  positions,  thereby  reducing  gross  income.  In  addition,  foreign
currency  markets may be extremely volatile from time to time. There also can be
no assurance  that  a  liquid  secondary market  will  exist  for  any  contract
purchased  or sold, and  a Series may  be required to  maintain a position until
exercise or  expiration,  which could  result  in  losses. The  SAI  contains  a
description  of the nature and trading  mechanics of options, Futures Contracts,
Options  on  Futures  Contracts,  Forward  Contracts  and  Options  on   Foreign
Currencies,  and  includes a  discussion of  the  risks related  to transactions
therein.
 
                                       10
<PAGE>
    Transactions  in  Forward  Contracts  may  be  entered  into  only  in   the
over-the-counter  market. Futures Contracts and Options on Futures Contracts may
be entered into  on U.S. exchanges  regulated by the  Commodity Futures  Trading
Commission  and on  foreign exchanges. In  addition, the  securities and indexes
underlying options, Futures Contracts and Options on Futures Contracts traded by
the Series will include both domestic and foreign securities.
 
    LOWER RATED BONDS: The  Series may invest in  fixed income securities  rated
Baa  by Moody's or BBB by S&P  or Fitch and comparable unrated securities. These
securities, while  normally  exhibiting  adequate  protection  parameters,  have
speculative   characteristics  and  changes  in  economic  conditions  or  other
circumstances are more likely to lead  to a weakened capacity to make  principal
and interest payments than in the case of higher grade securities.
 
    The  Series may also invest in securities rated Ba or lower by Moody's or BB
or lower by S&P  or Fitch and comparable  unrated securities (commonly known  as
"junk  bonds") to the extent described above.  See Appendix A to this Prospectus
for a description of these ratings. These securities are considered  speculative
and,  while generally providing greater income  than investments in higher rated
securities, will involve  greater risk  of principal and  income (including  the
possibility  of default or bankruptcy of the issuers of such securities) and may
involve greater  volatility  of price  (especially  during periods  of  economic
uncertainty or change) than securities in the higher rating categories. However,
since  yields vary over time,  no specific level of  income can ever be assured.
These lower  rated  high yielding  fixed  income securities  generally  tend  to
reflect economic changes and short-term corporate and industry developments to a
greater   extent  than  higher   rated  securities  which   react  primarily  to
fluctuations in the general level of interest rates (although these lower  rated
fixed  income securities  are also  affected by  changes in  interest rates, the
market's perception  of  their credit  quality,  and the  outlook  for  economic
growth).  In the past, economic downturns or an increase in interest rates have,
under certain circumstances, caused a higher incidence of default by the issuers
of these securities  and may  do so  in the future,  especially in  the case  of
highly  leveraged  issuers. During  certain periods,  the  higher yields  on the
Series' lower rated  high yielding  fixed income securities  are paid  primarily
because of the increased risk of loss of principal and income, arising from such
factors as the heightened possibility of default or bankruptcy of the issuers of
such  securities.  Due to  the fixed  income payments  of these  securities, the
Series may continue  to earn the  same level  of interest income  while its  net
asset  value declines due to portfolio losses, which could result in an increase
in the Series' yield despite the actual loss of principal. The market for  these
lower  rated fixed  income securities  may be  less liquid  than the  market for
investment grade  fixed income  securities, and  judgment may  at times  play  a
greater  role in valuing these  securities than in the  case of investment grade
fixed income securities. Changes in the value of securities subsequent to  their
acquisition  will not affect cash income or  yield to maturity to the Series but
will be reflected in the  net asset value of shares  of the Series. See the  SAI
for more information on lower rated securities.
 
    FOREIGN   SECURITIES:  The  Series  may  invest  in  dollar-denominated  and
non-dollar/denominated foreign securities.  Investing in  securities of  foreign
issuers  generally involves  risks not  ordinarily associated  with investing in
securities of  domestic  issuers.  These  include  changes  in  currency  rates,
exchange   control  regulations,  governmental  administration  or  economic  or
monetary policy (in  the U.S. or  abroad) or circumstances  in dealings  between
nations.  Costs may be  incurred in connection  with conversions between various
currencies. Special  considerations may  also include  more limited  information
about  foreign issuers,  higher brokerage costs,  different accounting standards
and thinner trading markets. Foreign securities markets may also be less liquid,
more volatile and  less subject  to government  supervision than  in the  United
States.  Investments in  foreign countries  could be  affected by  other factors
including expropriation,  confiscatory taxation  and potential  difficulties  in
enforcing  contractual obligations and  could be subject  to extended settlement
periods. The  Series  may hold  foreign  currency received  in  connection  with
investments in foreign securities when, in the judgment of the Adviser, it would
be  beneficial to convert such currency into U.S. dollars at a later date, based
on anticipated changes in the relevant  exchange rate. The Series may also  hold
foreign  currency in anticipation of purchasing  foreign securities. See the SAI
for further  discussion  of  foreign  securities  and  the  holding  of  foreign
currency, as well as the associated risks.
 
    AMERICAN  DEPOSITARY  RECEIPTS:  The Series  may  invest in  ADRs  which are
certificates issued  by a  U.S.  depository (usually  a  bank) and  represent  a
specified  quantity of shares of an underlying  non-U.S. stock on deposit with a
custodian bank as collateral. Because  ADRs trade on U.S. securities  exchanges,
the Adviser does not treat them as foreign securities. However, they are subject
to many of the risks of foreign securities such as changes in exchange rates and
more limited information about foreign issuers.
 
                                       11
<PAGE>
    EMERGING  MARKET SECURITIES: The  Series may invest  in emerging markets. In
addition to the general risks of investing in foreign securities, investments in
emerging markets involve special risks.  Securities of many issuers in  emerging
markets  may  be less  liquid and  more volatile  than securities  of comparable
domestic issuers.  These securities  may be  considered speculative  and,  while
generally offering higher income and the potential for capital appreciation, may
present   significantly  greater  risk.  Emerging  markets  may  have  different
clearance and  settlement procedures,  and in  certain markets  there have  been
times  when  settlements  have been  unable  to  keep pace  with  the  volume of
securities transactions,  making  it  difficult to  conduct  such  transactions.
Delays  in settlement could  result in temporary  periods when a  portion of the
assets of  the  Series  is uninvested  and  no  return is  earned  thereon.  The
inability  of the Series  to make intended security  purchases due to settlement
problems could cause  the Series  to miss  attractive investment  opportunities.
Inability  to dispose of  portfolio securities due  to settlement problems could
result in  losses to  the Series  due to  subsequent declines  in value  of  the
portfolio  securities,  a decrease  in  the level  of  liquidity in  the Series'
portfolio, or if the Series  has entered into a  contract to sell the  security,
possible  liability to  the purchaser. Certain  markets may  require payment for
securities before delivery, and in such  markets the Series bears the risk  that
the  securities will not be delivered and  that the Series' payments will not be
returned. Securities  prices  in  emerging markets  can  be  significantly  more
volatile than in the more developed nations of the world, reflecting the greater
uncertainties  of  investing  in  less  established  markets  and  economies. In
particular,  countries  with  emerging  markets  may  have  relatively  unstable
governments,  present the risk of nationalization of businesses, restrictions on
foreign ownership, or prohibitions of repatriation of assets, and may have  less
protection  of property rights  than more developed  countries. The economies of
countries with  emerging  markets may  be  predominantly  based on  only  a  few
industries,  may  be  highly vulnerable  to  changes  in local  or  global trade
conditions, and may suffer from extreme  and volatile debt burdens or  inflation
rates.  Local securities markets may trade a  small number of securities and may
be unable to  respond effectively  to increases in  trading volume,  potentially
making  prompt liquidation  of substantial  holdings difficult  or impossible at
times. Securities of issuers located in countries with emerging markets may have
limited marketability and may be subject to more abrupt or erratic movements.
 
    Certain  emerging  markets  may   require  governmental  approval  for   the
repatriation  of  investment  income,  capital  or  the  proceeds  of  sales  of
securities by foreign investors.  In addition, if a  deterioration occurs in  an
emerging  market's balance  of payments  or for  other reasons,  a country could
impose temporary restrictions on foreign  capital remittances. The Series  could
be  adversely  affected  by delays  in,  or  a refusal  to  grant,  any required
governmental approval for repatriation of capital, as well as by the application
to the Series of any restrictions on investments.
 
    Investment in  certain  foreign  emerging market  debt  obligations  may  be
restricted  or controlled to varying degrees. These restrictions or controls may
at times preclude investment in certain foreign emerging market debt obligations
and increase the expenses of a Series.
                              -------------------
 
SHORT-TERM INVESTMENTS FOR  TEMPORARY DEFENSIVE  PURPOSES --  During periods  of
unusual market conditions when the Adviser believes that investing for temporary
defensive  purposes is appropriate,  or in order  to meet anticipated redemption
requests, a large portion or all of the assets of the Series may be invested  in
cash (including foreign currency) or cash equivalents including, but not limited
to,   obligations  of   banks  (including  certificates   of  deposit,  bankers'
acceptances,  time  deposits  and  repurchase  agreements),  commercial   paper,
short-term notes, U.S. Government Securities and related repurchase agreements.
 
PORTFOLIO TRADING
 
    The Series intends to manage its portfolio by buying and selling securities,
as  well  as  holding securities  to  maturity,  to help  attain  its investment
objectives and policies.
 
    The Series will engage  in portfolio trading if  it believes a  transaction,
net  of  costs  (including  custodian  charges),  will  help  in  attaining  its
investment objectives. In trading portfolio securities, the Series seeks to take
advantage of  market  developments,  yield disparities  and  variations  in  the
creditworthiness  of issuers. For  a description of the  strategies which may be
used by the Series in trading portfolio securities, see "Portfolio  Transactions
and Brokerage Commissions" in the SAI.
 
    The  primary consideration  in placing portfolio  security transactions with
broker-dealers for execution  is to  obtain, and maintain  the availability  of,
execution  at  the  most  favorable  prices and  in  the  most  effective manner
possible. Consistent with
 
                                       12
<PAGE>
the foregoing primary consideration, the Rules of Fair Practice of the  National
Association  of Securities Dealers, Inc. (the "NASD") and such other policies as
the Trustees  of the  Trust may  determine, the  Adviser may  consider sales  of
Contracts  for which the Trust  is an investment option,  together with sales of
shares of other investment company clients  of MFS Fund Distributors, Inc.,  the
distributor  of shares of the Trust and of  the MFS Family of Funds, as a factor
in  the  selection   of  broker-dealers   to  execute   the  Series'   portfolio
transactions.  From  time  to  time the  Adviser  may  direct  certain portfolio
transactions to broker-dealer firms which, in turn, have agreed to pay a portion
of the Series' operating  expenses (e.g., fees charged  by the custodian of  the
Series' assets). For a further discussion of portfolio trading, see the SAI.
                              -------------------
 
    The  SAI includes a  discussion of other investment  policies and listing of
specific investment restrictions  which govern  the investment  policies of  the
Series.  The specific investment  restrictions listed in the  SAI may be changed
without shareholder  approval  unless indicated  otherwise  (see the  SAI).  The
Series'  investment limitations, policies and rating standards 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.
 
7.  MANAGEMENT OF THE SERIES
 
    The   Trust's  Board  of  Trustees,  as   part  of  its  overall  management
responsibility, oversees  various  organizations responsible  for  each  Series'
day-to-day management.
 
INVESTMENT  ADVISER -- MFS manages the Series pursuant to an Investment Advisory
Agreement with the  Trust on  behalf of  the Series  dated April  14, 1994  (the
"Advisory  Agreement"). Under  the Advisory  Agreement, MFS  provides the Series
with overall  investment advisory  services.  Subject to  such policies  as  the
Trustees  may determine, MFS makes investment  decisions for the Series. For its
services and  facilities,  MFS receives  a  management fee,  computed  and  paid
monthly,  in an  amount equal to  0.75% of the  average daily net  assets of the
Series:
 
    For the  fiscal year  ended  December 31,  1996,  MFS received  $314,262  in
management fees from the Series under the Advisory Agreement and assumed $62,962
of the Series' expenses.
 
    The  identity and background of the portfolio managers for the Series is set
forth below. Unless  indicated otherwise,  each portfolio manager  has acted  in
that capacity since the commencement of investment operations of the Series.
 
<TABLE>
<CAPTION>
SERIES                                                         PORTFOLIO MANAGERS
- -----------------------------  -----------------------------------------------------------------------------------
<S>                            <C>
Emerging Growth Series         John W. Ballen, a Senior Vice President of MFS, has been employed by the Adviser as
                               a  portfolio manager since 1984. Toni Y. Shimura, a Vice President of MFS, has been
                               employed by the Adviser  as a portfolio  manager since 1987.  Ms. Shimura became  a
                               portfolio manager of the Series on November 30, 1995.
</TABLE>
 
    MFS  also serves as investment adviser to each of the other funds in the MFS
Family of Funds  (the "MFS  Funds") and to  MFS-Registered Trademark-  Municipal
Income Trust, MFS Multimarket Income Trust, MFS Government Markets Income Trust,
MFS  Intermediate  Income Trust,  MFS Charter  Income  Trust, MFS  Special Value
Trust, MFS Institutional Trust,  MFS Union Standard  Trust, MFS/Sun Life  Series
Trust,  and seven  variable accounts, each  of which is  a registered investment
company established by Sun Life Assurance Company of Canada (U.S.) ("Sun Life of
Canada (U.S.)") in connection  with the sale  of various fixed/variable  annuity
contracts.  MFS  and its  wholly owned  subsidiary, MFS  Institutional Advisers,
Inc., provide investment advice to substantial private clients.
 
    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 in the United States, Massachusetts  Investors
Trust.   Net  assets  under   the  management  of   the  MFS  organization  were
approximately $52.8  billion on  behalf of  approximately 2.3  million  investor
accounts  as of February 28, 1997. As of such date, the MFS organization managed
approximately  $28.9  billion  of  assets  invested  in  equity  securities  and
approximately  $19.9  billion of  assets  invested in  fixed  income securities.
Approximately $4.0  billion  of  the  assets managed  by  MFS  are  invested  in
securities of foreign issuers and non-U.S. dollar-denominated securities of U.S.
issuers.  MFS is a subsidiary of  Sun Life of Canada (U.S.),  which in turn is a
wholly owned subsidiary of  Sun Life Assurance Company  of Canada ("Sun  Life").
The  Directors of MFS are A. Keith  Brodkin, Jeffrey L. Shames, Arnold D. Scott,
John D. McNeil and Donald A. Stewart. Mr. Brodkin is the Chairman, Mr. Shames is
the President  and  Mr. Scott  is  the Secretary  and  a Senior  Executive  Vice
President of MFS. Messrs. McNeil
 
                                       13
<PAGE>
and Stewart are the Chairman and President, respectively, of Sun Life. Sun Life,
a  mutual  life insurance  company,  is one  of  the largest  international life
insurance companies and  has been  operating in  the United  States since  1895,
establishing  a headquarters office here in  1973. The executive officers of MFS
report to the Chairman of Sun Life.
 
    A. Keith Brodkin, the Chairman  and a Director of  MFS, is the Chairman  and
President  and a Trustee of the Trust. W. Thomas London, Stephen E. Cavan, James
R. Bordewick, Jr.,  and James  O. Yost,  all of whom  are officers  of MFS,  are
officers of the Trust.
 
    MFS  has established a strategic alliance with Foreign & Colonial Management
Ltd. ("Foreign & Colonial"). Foreign  & Colonial is a  subsidiary of two of  the
world's  oldest  financial  services institutions,  the  London-based  Foreign &
Colonial Investment Trust PLC, which pioneered the idea of investment management
in 1868, and HYPO-BANK (Bayerische Hypotheken-und Weschsel-Bank AG), the  oldest
publicly  listed bank in Germany, founded in 1835. As part of this alliance, the
portfolio managers and investment analysts of  MFS and Foreign & Colonial  share
their  views on  a variety  of investment related  issues, such  as the economy,
securities  markets,  portfolio   securities  and   their  issuers,   investment
recommendations,  strategies and  techniques, risk  analysis, trading strategies
and other  portfolio  management  matters.  MFS  has  access  to  the  extensive
international  equity investment expertise of Foreign  & Colonial, and Foreign &
Colonial has access to  the extensive U.S. equity  investment expertise of  MFS.
MFS  and Foreign Colonial each have investment personnel working in each other's
offices in Boston and London, respectively.
 
    In certain instances  there may  be securities  which are  suitable for  the
Series'  portfolio as well as for portfolios  of other clients of MFS or clients
of Foreign  &  Colonial.  Some simultaneous  transactions  are  inevitable  when
several  clients  receive investment  advice from  MFS  and Foreign  & Colonial,
particularly when the same security is suitable for more than one client.  While
in  some cases this arrangement could have  a detrimental effect on the price or
availability of the security as far as the Series is concerned, in other  cases,
however, it may produce increased investment opportunities for the Series.
 
    From  time to time, the Adviser may  purchase, redeem and exchange shares of
any Series. The purchase  by the Adviser  of shares of the  Series may have  the
effect  of  lowering the  Series'  expense ratio,  while  the redemption  by the
Adviser of shares of the  Series may have the  effect of increasing the  Series'
expense ratio.
 
    DISTRIBUTOR   --  MFS  Fund  Distributors,  Inc.  ("MFD"),  a  wholly  owned
subsidiary of MFS, is the distributor of shares of the Series and also serves as
distributor for certain of the other mutual funds managed by MFS.
 
    ADMINISTRATOR  --  MFS  provides  the  Series  with  certain  administrative
services  pursuant to a Master Administrative  Services Agreement dated March 1,
1997. Under  this Agreement,  MFS provides  the Series  with certain  financial,
legal, compliance, shareholder communications and other administrative services.
As  a partial reimbursement for the cost of providing these services, the Series
pays MFS an administrative  fee up to  0.015% per annum  of the Series'  average
daily net assets, provided that the administrative fee is not assessed on Series
assets that exceed $3 billion.
 
    SHAREHOLDER  SERVICING AGENT --  MFS Service Center,  Inc. (the "Shareholder
Servicing Agent"), a wholly owned  subsidiary of MFS, performs transfer  agency,
certain dividend disbursing agency and other services for the Series.
 
8.  INFORMATION CONCERNING SHARES OF EACH SERIES
 
PURCHASES AND REDEMPTIONS
 
    The  separate accounts of the Participating Insurance Companies place orders
to purchase and redeem shares  of the Series based  on, among other things,  the
amount of premium payments to be invested and surrender and transfer requests to
be  effected on that day pursuant to Contracts. Orders received by the Trust are
effected on days on which the Exchange is open for trading. For orders  received
by  the Trust before  the close of  regular trading on  the Exchange (normally 4
p.m. eastern time), such purchases and  redemptions of the shares of the  Series
are  effected at the respective  net asset value per  share determined as of the
close of  regular  trading on  the  Exchange  on that  same  day.  Participating
Insurance  Companies shall be the designee of  the Trust for receipt of purchase
and redemption orders from Contract holders  and receipt by such designee  shall
constitute receipt by the Trust; provided that the Trust receives notice of such
order  by 9:30 a.m. eastern time on the next following day on which the Exchange
is open for trading. Payment for shares shall be by federal funds transmitted by
wire and must be received by 2:00 p.m. eastern time on the next following day on
which the Exchange  is open for  trading after the  purchase order is  received.
Redemption proceeds shall be by federal
 
                                       14
<PAGE>
funds  transmitted by wire  and shall be sent  by 2:00 p.m.  eastern time on the
next following  day  on  which  the  Exchange is  open  for  trading  after  the
redemption  order  is received.  No fee  is charged  the shareholders  when they
redeem Series shares.
 
    The offering of shares of the Series  may be suspended for a period of  time
and  the  Series  reserves the  right  to  refuse any  specific  purchase order.
Purchase orders may be refused if, in the Adviser's opinion, they are of a  size
that would disrupt the management of the Series. The Trust may suspend the right
of  redemption of shares of the Series  and may postpone payment for any period:
(i) during which the Exchange is closed other than customary weekend and holiday
closings or during which  trading on the Exchange  is restricted; (ii) when  the
SEC  determines  that a  state of  emergency  exists which  may make  payment or
transfer not reasonably practicable;  (iii) as the SEC  may by order permit  for
the  protection of the security  holders of the Trust; or  (iv) at any time when
the Trust may, under applicable laws, rules and regulations, suspend payment  on
the redemption of its shares.
 
    Should  any conflict between Contract holders arise which would require that
a substantial  amount  of net  assets  be  withdrawn from  the  Series,  orderly
portfolio  management  could be  disrupted to  the  potential detriment  of such
Contract.
 
NET ASSET VALUE
 
    The net asset value per  share of the Series  is determined each day  during
which  the Exchange is open for trading.  This determination is made once during
each such day as of  the close of regular trading  on the Exchange by  deducting
the  amount of the Series' liabilities from  the value of the Series' assets and
dividing the  difference by  the number  of shares  of the  Series  outstanding.
Values  of assets in the Series' portfolio  are determined on the basis of their
market or other fair value, as described in the SAI. All investments, assets and
liabilities are expressed in U.S.  dollars based upon current currency  exchange
rates.
 
DISTRIBUTIONS
 
    Substantially all of the Series' net investment income for any calendar year
is  declared as dividends and paid to its shareholders as dividends on an annual
basis. In addition,  the Series may  make one or  more distributions during  the
calendar year to its shareholders from any long-term capital gains, and may also
make  one  or more  distributions to  its  shareholders from  short-term capital
gains. In determining the net investment income available for distribution,  the
Series  may rely on projections of  its anticipated net investment income (which
may include  short-term capital  gains from  the sales  of securities  or  other
assets,  and, if allowed  by the Series'  investment restrictions, premiums from
options written), over  a longer  term, rather  than its  actual net  investment
income for the period.
 
    Shareholders  of the Series may elect  to receive dividends and capital gain
distributions in either cash or additional shares.
 
TAX STATUS
 
    Each Series of the Trust is treated as a separate entity for federal  income
tax  purposes. In  order to  minimize the taxes  each Series  would otherwise be
required to  pay, each  Series intends  to  qualify each  year as  a  "regulated
investment  company" under Subchapter M of the Internal Revenue Code of 1986, as
amended ("the Code"). Because each Series  intends to distribute all of its  net
investment  income and net capital gains  to its shareholders in accordance with
the timing requirements imposed by the Code, it is not expected that any of  the
Series will be required to pay entity level federal income or excise taxes.
 
    Shares  of  the  Series  are offered  only  to  the  Participating Insurance
Companies' separate accounts  that fund Contracts.  See the applicable  Contract
prospectus  for a  discussion of  the federal  income tax  treatment of  (1) the
separate accounts that purchase  and hold Series shares  and (2) the holders  of
the  Contracts  that  are funded  through  those  accounts. In  addition  to the
diversification requirements  of Subchapter  M  of the  Code, each  Series  also
intends  to diversify its assets as required  by Code Section 817(h)(1), and the
regulations thereunder. See also "Tax Status" in the SAI.
 
DESCRIPTION OF SHARES, VOTING RIGHTS AND LIABILITIES
 
    The Series currently has one class of shares, entitled Shares of  Beneficial
Interest  (without par value).  The Trust has  reserved the right  to create and
issue additional  classes and  series of  shares, in  which case  each class  of
shares  of a  series would  participate equally  in the  earnings, dividends and
assets attributable to that  class of that  particular series. Shareholders  are
entitled  to one vote for each share held, and shares of the Series are entitled
to vote  separately to  approve  investment advisory  agreements or  changes  in
investment  restrictions with  respect to the  Series, but shares  of the Series
vote together  in  the  election  of  Trustees  and  selection  of  accountants.
Additionally, the Series will vote separately on any
 
                                       15
<PAGE>
other  matter that affects  solely the Series, but  will otherwise vote together
with all other Series on  all other matters. The Trust  does not intend to  hold
annual  shareholder meetings. The  Declaration of Trust  provides that a Trustee
may be removed  from office in  certain instances. See  "Description of  Shares,
Voting Rights and Liabilities" in the SAI.
 
    Each  share of the Series represents  an equal proportionate interest in the
Series with each share, subject to the liabilities of the Series. Shares have no
pre-emptive or  conversion rights.  Shares are  fully paid  and  non-assessable.
Should  the Series be liquidated, shareholders are entitled to share PRO RATA in
the net assets available for distribution to shareholders. Shares will remain on
deposit with  the  Shareholder Servicing  Agent  and certificates  will  not  be
issued.
 
    The  Trust  is an  entity of  the  type commonly  known as  a "Massachusetts
business trust."  Under Massachusetts  law, shareholders  of such  a trust  may,
under  certain  circumstances, be  held personally  liable  as partners  for its
obligations. However,  the risk  of a  shareholder incurring  financial loss  on
account  of  shareholder liability  is limited  to  circumstances in  which both
inadequate insurance existed (E.G., fidelity bonding and omission insurance) and
the Trust itself was unable to meet its obligations.
 
PERFORMANCE INFORMATION
 
    The Series' performance may be quoted  in advertising in terms of yield  and
total  return. Performance is based on historical results and is not intended to
indicate future  performance. Performance  quoted for  the Series  includes  the
effect  of  deducting the  Series'  expenses, but  may  not include  charges and
expenses attributable  to  any  particular insurance  product.  Excluding  these
charges  from quotations of the Series' performance has the effect of increasing
the performance quoted.  Performance for the  Series will vary  based on,  among
other  things, changes in market conditions, the level of interest rates and the
level of  the  Series'  expenses.  For further  information  about  the  Series'
performance  for the fiscal year ended December 31, 1996, please see the Series'
Annual Report. A copy of  this Annual Report may  be obtained without charge  by
contacting the Shareholder Servicing Agent (see back cover for address and phone
number).
 
    From  time to time, quotations of the  Series' total return and yield may be
included in  advertisements,  sales literature  or  reports to  shareholders  or
prospective  investors. The total return of the Series refers to return assuming
an investment has been held in the Series  for one year and for the life of  the
Series  (the ending date of  which will be stated).  The total return quotations
may be expressed in terms  of average annual or  cumulative rates of return  for
all  periods quoted.  Average annual total  return refers to  the average annual
compound rate of return of an investment in the Series. Cumulative total  return
represents  the cumulative change in value of  an investment in the Series. Both
will assume that all dividends and capital gains distributions were  reinvested.
The  yield of the Series refers to net investment income generated by the Series
over a specified 30-day (or one month) period. This income is then "annualized."
That is, the amount of income generated by the Series during that 30-day (or one
month) period is assumed to be generated over a 12-month period and is shown  as
a percentage of net asset value.
 
EXPENSES
 
    The  Trust pays the compensation of the Trustees who are not officers of MFS
and all expenses of the Series (other  than those assumed by MFS) including  but
not  limited to: governmental fees; interest  charges; taxes; membership dues in
the Investment Company Institute allocable to  the Series; fees and expenses  of
independent  auditors, of legal counsel, and of any transfer agent, registrar or
dividend disbursing agent of the Series; expenses of repurchasing and  redeeming
shares  and servicing shareholder accounts;  expenses of preparing, printing and
mailing  prospectuses,  periodic  reports,  notices  and  proxy  statements   to
shareholders  and to governmental officers  and commissions; brokerage and other
expenses connected with  the execution,  recording and  settlement of  portfolio
security transactions; insurance premiums; fees and expenses of Investors Bank &
Trust  Company, the Trust's Custodian, for all services to the Series, including
safekeeping of funds and securities and maintaining required books and accounts;
expenses of  calculating  the net  asset  value of  shares  of the  Series;  and
expenses   of  shareholder   meetings.  Expenses   relating  to   the  issuance,
registration and  qualification of  shares of  the Series  and the  preparation,
printing  and mailing of  prospectuses are borne  by the Series  except that the
Distribution Agreement with MFD requires MFD to pay for prospectuses that are to
be used for sales purposes. Expenses of the Trust which are not attributable  to
a  specific Series  are allocated  between the  Series in  a manner  believed by
management of the Trust to be fair and equitable.
 
    Subject to termination or  revision at the sole  discretion of MFS, MFS  has
agreed  to bear expenses of  the Series such that  the Series' "Other Expenses,"
which are defined to  include all expenses of  the Series except for  management
fees,  do not exceed  0.25% of the average  daily net assets  of the Series (the
"Maximum Percentage"). The  obligation of  MFS to  bear these  expenses for  the
Series terminates on the last day of the Series' fiscal year in which its "Other
Expenses" are
 
                                       16
<PAGE>
less than or equal to the Maximum Percentage. The payments made by MFS on behalf
of  the Series under this arrangement are subject to reimbursement by the Series
to MFS, which will  be accomplished by the  payment of an expense  reimbursement
fee  by the  Series to  MFS computed  and paid  monthly at  a percentage  of the
Series' average  daily net  assets for  its  then current  fiscal year,  with  a
limitation that immediately after such payment the Series' "Other Expenses" will
not  exceed the Maximum Percentage. This  expense reimbursement by the Series to
MFS terminates on the earlier of the  date on which payments made by the  Series
equal  the prior payment  of such reimbursable  expenses by MFS  or December 31,
2004.
 
SHAREHOLDER COMMUNICATIONS
 
    Owners of Contracts  issued by Participating  Insurance Companies for  which
shares  of one or more  Series are the investment  vehicle will receive from the
Participating Insurance Companies semi-annual  financial statements and  audited
year-end  financial statements  certified by  the Trust's  independent certified
public accountants. Each report will show the investments owned by the Trust and
the valuations thereof  as determined  by the  Trustees and  will provide  other
information about the Trust and its operations.
 
    Participating  Insurance Companies  with inquiries  regarding the  Trust may
call the Trust's Shareholder  Servicing Agent. (See back  cover for address  and
phone number.)
                              -------------------
 
    The  SAI for the Trust,  dated May 1, 1997,  as amended or supplemented from
time to  time, contains  more detailed  information about  each of  the  Series,
including  information related to: (i)  the investment policies and restrictions
of each Series; (ii) the Trustees, officers and investment adviser of the Trust;
(iii) portfolio transactions; (iv) the  shares of each Series, including  rights
and  liabilities of  shareholders; (v)  the method  used to  calculate yield and
total rate of return  quotations of each Series;  (vi) the determination of  net
asset  value  of shares  of  each Series;  and  (vii) certain  voting  rights of
shareholders of each Series.
 
                                       17
<PAGE>
                                                                      APPENDIX A
 
                          DESCRIPTION OF BOND RATINGS
 
The ratings of Moody's, S&P and Fitch represent their opinions as to the quality
of  various debt instruments. It should be emphasized, however, that ratings are
not absolute standards of quality. Consequently, debt instruments with the  same
maturity,  coupon and rating may have different yields while debt instruments of
the same maturity and coupon with different ratings may have the same yield.
 
                        MOODY'S INVESTORS SERVICE, INC.
 
    AAA: Bonds which are rated  Aaa are judged to be  of the best quality.  They
carry  the smallest degree of  investment risk and are  generally referred to as
"gilt edged." Interest payments are protected by a large or by an  exceptionally
stable margin and principal is secure. While the various protective elements are
likely  to change, such changes as can be visualized are most unlikely to impair
the fundamentally strong position of such issues.
 
    AA: Bonds  which are  rated Aa  are  judged to  be of  high quality  by  all
standards. Together with the Aaa group they comprise what are generally known as
high  grade bonds. They are  rated lower than the  best bonds because margins of
protection may  not  be  as  large  as in  Aaa  securities  or  fluctuations  of
protective  elements may be of greater amplitude  or there may be other elements
present which  make the  long-term  risks appear  somewhat  larger than  in  Aaa
securities.
 
    A:  Bonds which are rated A possess many favorable investment attributes and
are to be considered as upper-medium-grade obligations. Factors giving  security
to  principal and interest are considered  adequate, but elements may be present
which suggest a susceptibility to impairment sometime in the future.
 
    BAA: Bonds which are  rated Baa are  considered as medium-grade  obligations
(I.E.,  they are neither highly protected nor poorly secured). Interest payments
and principal security appear  adequate for the  present but certain  protective
elements  may be lacking or may  be characteristically unreliable over any great
length of time. Such  bonds lack outstanding  investment characteristics and  in
fact have speculative characteristics as well.
 
    BA:  Bonds which are rated Ba are judged to have speculative elements; their
future cannot be considered  as well-assured. Often  the protection of  interest
and  principal payments  may be very  moderate and thereby  not well safeguarded
during both  good  and  bad  times over  the  future.  Uncertainty  of  position
characterizes bonds in this class.
 
    B:  Bonds which are rated B  generally lack characteristics of the desirable
investment. Assurance of interest  and principal payments  or of maintenance  of
other terms of the contract over any long period of time may be small.
 
    CAA:  Bonds which are rated Caa are of  poor standing. Such issues may be in
default or there may be present elements of danger with respect to principal  or
interest.
 
    CA:  Bonds which are rated Ca represent obligations which are speculative in
a  high  degree.  Such  issues  are  often  in  default  or  have  other  marked
shortcomings.
 
    C: Bonds which are rated C are the lowest rated class of bonds and issues so
rated  can be regarded as having extremely  poor prospects of ever attaining any
real investment standing.
 
    ABSENCE OF RATING: Where no rating has  been assigned or where a rating  has
been  suspended or withdrawn, it may be  for reasons unrelated to the quality of
the issue.
 
    Should no rating be assigned, the reason may be one of the following:
 
    1. an application for rating was not received or accepted;
 
    2. the issue or issuer  belongs to a group  of securities or companies  that
       are not rated as a matter of policy;
 
    3. there is a lack of essential data pertaining to the issue or issuer; or
 
    4. the issue was privately placed, in which case the rating is not published
       in Moody's publications.
 
    Suspension  or withdrawal may occur if new and material circumstances arise,
the effects  of which  preclude satisfactory  analysis; if  there is  no  longer
available  reasonable up-to-date data  to permit a  judgment to be  formed; if a
bond is called for redemption; or for other reasons.
 
                                      A-1
<PAGE>
                       STANDARD & POOR'S RATINGS SERVICES
 
    AAA: Debt rated AAA has the highest rating assigned by S&P. Capacity to  pay
interest and repay principal is extremely strong.
 
    AA:  Debt rated  AA has  a very  strong capacity  to pay  interest and repay
principal and differs from the highest rated issues only in small degree.
 
    A: Debt rated A has a strong  capacity to pay interest and repay  principal,
although  it is somewhat more  susceptible to the adverse  effects of changes in
circumstances and economic conditions than debt in higher-rated categories.
 
    BBB: Debt  rated BBB  is regarded  as  having an  adequate capacity  to  pay
interest  and repay principal. Whereas  it normally exhibits adequate protection
parameters, adverse  economic  conditions  or changing  circumstances  are  more
likely  to lead to a  weakened capacity to pay  interest and repay principal for
debt in this category than in higher-rated categories.
 
    BB: Debt rated  BB has less  near-term vulnerability to  default than  other
speculative issues. However, it faces major ongoing uncertainties or exposure to
adverse  business,  financial,  or  economic  conditions  which  could  lead  to
inadequate capacity  to meet  timely  interest and  principal payments.  The  BB
rating  category  is also  used for  debt  subordinated to  senior debt  that is
assigned an actual or implied BBB- rating.
 
    B: Debt rated B has a greater vulnerability to default but currently has the
capacity to meet interest payments  and principal repayments. Adverse  business,
financial  or economic conditions will likely  impair capacity or willingness to
pay interest and repay principal.  The B rating category  is also used for  debt
subordinated  to senior  debt that is  assigned an  actual or implied  BB or BB-
rating.
 
    CCC: Debt rated CCC has  a currently identifiable vulnerability to  default,
and  is dependent upon favorable business,  financial and economic conditions to
meet timely payment  of interest  and repayment of  principal. In  the event  of
adverse  business, financial, or  economic conditions, it is  not likely to have
the capacity to  pay interest and  repay principal. The  CCC rating category  is
also  used for debt  subordinated to senior  debt that is  assigned an actual or
implied B or B- rating.
 
    CC: The rating CC is typically  applied to debt subordinated to senior  debt
that is assigned an actual or implied CCC rating.
 
    C:  The rating C  is typically applied  to debt subordinated  to senior debt
which is assigned an  actual or implied  CCC- debt rating. The  C rating may  be
used  to cover a situation where a  bankruptcy petition has been filed, but debt
service payments are continued.
 
    CI: The rating CI is reserved for income bonds on which no interest is being
paid.
 
    D: Debt rated D is  in payment default. The D  rating category is used  when
interest  payments or principal payments  are not made on  the date due, even if
the applicable  grace period  has not  expired, unless  S&P believes  that  such
payments  will be made during such grace period.  The D rating also will be used
upon  the  filing  of  a  bankruptcy  petition  if  debt  service  payments  are
jeopardized.
 
    PLUS  (+) OR MINUS  (-): The ratings from  AA to CCC may  be modified by the
addition of a  plus or minus  sign to  show relative standing  within the  major
rating categories.
 
    NR:  Indicates  that no  public  rating has  been  requested, that  there is
insufficient information on which to base a rating, or that S&P does not rate  a
particular type of obligation as a matter of policy.
 
A-1 AND P-1 COMMERCIAL PAPER RATINGS
 
Description of S&P, Fitch and Moody's highest commercial paper ratings:
 
    The  rating "A" is the  highest commercial paper rating  assigned by S&P and
Fitch, and issues  so rated  are regarded as  having the  greatest capacity  for
timely  payment. Issues in the "A" category are delineated with the numbers 1, 2
and 3 to indicate the relative  degree of safety. The A-1 designation  indicates
that  the degree  of safety regarding  timely payment is  either overwhelming or
very  strong.  Those  A-1  issues  determined  to  possess  overwhelming  safety
characteristics will be denoted with a plus (+) sign designation.
 
                                      A-2
<PAGE>
    The  rating P-1 is the highest  commercial paper rating assigned by Moody's.
Issuers rated P-1 have a superior ability for repayment. P-1 repayment  capacity
will  normally be evidenced by the following characteristics: (1) leading market
positions in well  established industries;  (2) high  rates of  return on  funds
employed;  (3) conservative  capitalization structure with  moderate reliance on
debt and ample asset protection; (4) broad margins in earnings coverage of fixed
financial charges and high  internal cash generation;  and (5) well  established
access  to  a  range  of  financial markets  and  assured  sources  of alternate
liquidity.
 
                         FITCH INVESTORS SERVICE, INC.
 
    AAA: Bonds  considered to  be investment  grade and  of the  highest  credit
quality.  The obligor  has an exceptionally  strong ability to  pay interest and
prepay principal, which  is unlikely  to be affected  by reasonably  foreseeable
events.
 
    AA: Bonds considered to be investment grade and of very high credit quality.
The  obligor's  ability to  pay  interest and  repay  principal is  very strong,
although not quite as strong  as bonds rated 'AAA'.  Because bonds rated in  the
'AAA' and 'AA' categories are not significantly vulnerable to foreseeable future
developments, short-term debt of these issuers is generally rated 'F-1+'.
 
    A:  Bonds considered to be investment grade and of very high credit quality.
The obligor's ability to  pay interest and repay  principal is considered to  be
strong, but may be more vulnerable to adverse changes in economic conditions and
circumstances than bonds with higher ratings.
 
    BBB:  Bonds considered  to be  investment grade  and of  satisfactory credit
quality. The obligor's ability to pay interest and repay principal is considered
to be  adequate.  Adverse  changes in  economic  conditions  and  circumstances,
however,  are more likely to have adverse  impact on these bonds and, therefore,
impair timely payment. The likelihood that the ratings of these bonds will  fall
below investment grade is higher than for bonds with higher ratings.
 
    BB:  Bonds are considered speculative. The obligor's ability to pay interest
and repay  principal may  be affected  over time  by adverse  economic  changes.
However,  business  and financial  alternatives  can be  identified  which could
assist the obligor in satisfying its debt service requirements.
 
    B: Bonds are considered  highly speculative. While bonds  in this class  are
currently meeting debt service requirements, the probability of continued timely
payment  of  principal and  interest reflects  the  obligor's limited  margin of
safety and the need for reasonable business and economic activity throughout the
life of the issue.
 
    CCC: Bonds have certain identifiable characteristics which, if not remedied,
may lead to default.  The ability to meet  obligations requires an  advantageous
business and economic environment.
 
    CC:  Bonds are  minimally protected. Default  in payment  of interest and/or
principal seems probable over time.
 
    C: Bonds are in imminent default in payment of interest of principal.
 
    PLUS(+) MINUS(-): Plus  and minus  signs are used  with a  rating symbol  to
indicate  the relative position of a credit  within the rated category. Plus and
minus signs, however, are not used in the 'AAA' category.
 
    NR: indicates that Fitch does not rate the specific issue.
 
    CONDITIONAL: A conditional rating is  premised on the successful  completion
of a project or the occurrence of a specific event.
 
    SUSPENDED:  A rating is suspended when Fitch deems the amount of information
available from the issuer to be inadequate for rating purposes.
 
    WITHDRAWN: A rating will be withdrawn when an issue matures or is called  or
refinanced,  and, at Fitch's discretion, when  an issuer fails to furnish proper
and timely information.
 
    FITCHALERT: Ratings  are placed  on  FitchAlert to  notify investors  of  an
occurrence  that is likely to result in a rating change and the likely direction
of such  change.  These are  designated  a "Positive,"  indicating  a  potential
upgrade,  "Negative," for potential downgrade,  or "Evolving," where ratings may
be lowered. FitchAlert is relatively short-term and should be resolved within 12
months.
 
                                      A-3
<PAGE>
                        DUFF & PHELPS CREDIT RATING CO.
 
    AAA: Bonds  considered to  be investment  grade and  of the  highest  credit
quality.  The obligor  has an exceptionally  strong ability to  pay interest and
repay principal,  which is  unlikely to  be affected  by reasonably  foreseeable
events.
 
    AA: Bonds considered to be investment grade and or very high credit quality.
The  obligor's  ability to  pay  interest and  repay  principal is  very strong,
although not quite as strong  as bonds rated 'AAA'.  Because bonds rated in  the
'AAA' and 'AA' categories are not significantly vulnerable to foreseeable future
developments, short-term debt of these issuers is generally rated 'D-1+'.
 
    A:  Bonds considered to be investment grade  and of high credit quality. The
obligor's ability  to pay  interest  and repay  principal  is considered  to  be
strong, but may be more vulnerable to adverse changes in economic conditions and
circumstances than bonds with higher ratings.
 
    BBB:  Bonds considered  to be  investment grade  and of  satisfactory credit
quality. The obligor's ability to pay interest and repay principal is considered
to be  adequate.  Adverse  changes in  economic  conditions  and  circumstances,
however,  are more likely to  have adverse impact on  these bonds, and therefore
impair timely payment. The likelihood that the ratings of these bonds will  fall
below investment grade is higher than for bonds with higher ratings.
 
    BB:  Bonds are considered speculative. The obligor's ability to pay interest
and repay  principal may  be affected  over time  by adverse  economic  changes.
However,  business,  and financial  alternatives can  be identified  which could
assist the obligor in satisfying its debt service requirements.
 
    B: Bonds are considered  highly speculative. While bonds  in this class  are
currently meeting debt service requirements, the probability of continued timely
payment  of  principal and  interest reflects  the  obligor's limited  margin of
safety and the need for reasonable business and economic activity throughout the
life of the issue.
 
    CCC: Bonds have certain identifiable characteristics which, if not remedied,
may lead to default.  The ability to meet  obligations requires an  advantageous
business and economic environment.
 
    PLUS (+) OR MINUS (-): Plus and minus signs are used with a rating symbol to
indicate  the relative position of  a credit within a  rating category. Plus and
minus signs, however, are not used in the 'AAA' category.
 
    NR: Indicates that Duff & Phelps does not rate the specific issue.
 
                        DUFF & PHELPS SHORT-TERM RATINGS
 
    D-1+: Highest certainty of  timely payment. Short-term liquidity,  including
internal  operation factors  and/or access to  alternative sources  of funds, is
outstanding  and  safety  is  just  below  risk-free  U.S.  Treasury  short-term
obligations.
 
    D-1:  Very high certainty of timely payment. Liquidity factors are excellent
and supported by good fundamental protection factors. Risk factors are minor.
 
    D-1-: High certainty  of timely  payment. Liquidity factors  are strong  and
supported by good fundamental protection factors. Risk factors are very small.
 
    D-2:  Good  certainty  of  timely  payment.  Liquidity  factors  and company
fundamentals are  sound.  Although  ongoing  funding  needs  may  enlarge  total
financing  requirements, access  to capital  markets is  good. Risk  factors are
small.
 
    D-3: Satisfactory liquidity and other  protection factors qualify issues  as
to  investment grade.  Risk factors  are larger  and subject  to more variation.
Nevertheless, timely payment is expected.
 
    D-4: Speculative investment characteristics. Liquidity is not sufficient  to
insure  against disruption in debt service.  Operating factors and market access
may be subject to a high degree of variation.
 
    D-5: Issuer failed to meet scheduled principal and/or interest payments.
 
                                      A-4
<PAGE>
INVESTMENT ADVISER
Massachusetts Financial Services Company
500 Boylston Street, Boston, MA 02116
(617) 954-5000
(800) 637-8730
 
DISTRIBUTOR
MFS Fund Distributors, Inc.
500 Boylston Street, Boston, MA 02116
(617) 954-5000
CUSTODIAN
Investors Bank & Trust Company
89 South Street, Boston, MA 02111
 
DIVIDEND DISBURSING AGENT
State Street Bank and Trust Company
225 Franklin Street, Boston, MA 02110
 
SHAREHOLDER SERVICING AGENT
MFS Service Center, Inc.
500 Boylston Street, Boston, MA 02116
Toll free: (800) 343-2829, Ext. 3500
MAILING ADDRESS:
P.O. Box 1400, Boston, MA 02104-9985
INDEPENDENT AUDITORS
Deloitte & Touche LLP
125 Summer Street, Boston, MA 02110
 
                      ------------------------------------
                           MFS-REGISTERED TRADEMARK-
                                    EMERGING
                                     GROWTH
                                     SERIES
 
                                     [LOGO]
 
                                   PROSPECTUS
 
                                  MAY 1, 1997
 
                                     [LOGO]
                MFS-REGISTERED TRADEMARK- EMERGING GROWTH SERIES
                     500 Boylston Street, Boston, MA 02116
 
                            ------------------------


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