<PAGE>
MFS(R) INSTITUTIONAL MID CAP GROWTH FUND
Supplement to the Current Prospectus
The description of portfolio managers under the "Management of the Funds"
section is hereby restated as follows:
Mark Regan, a Senior Vice President of MFS, has been employed in the investment
management area of the adviser since 1989 and has been the portfolio manager of
the fund since the fund's inception in December 1995. David E. Sette-Ducati, a
Vice President of MFS, has been employed in the investment management area of
the adviser since 1995. Mr. Sette-Ducati became a portfolio manager of the fund
effective May 1, 2000. John W. Ballen, President and Chief Investment Officer of
MFS, has provided oversight of the fund's management since December 1995. Mr.
Ballen has been employed in the investment management area of the adviser since
1984.
The date of this Supplement is May 1, 2000.
