<PAGE>
SUPPLEMENT DATED JULY 27, 2000
TO PROSPECTUS DATED MAY 1, 2000
OF
EQUITY GROWTH PORTFOLIO
FOCUS EQUITY PORTFOLIO
SMALL COMPANY GROWTH PORTFOLIO
VALUE EQUITY PORTFOLIO
TECHNOLOGY PORTFOLIO
PORTFOLIOS OF THE
MORGAN STANLEY DEAN WITTER INSTITUTIONAL FUND, INC.
P.O. BOX 2798
BOSTON, MASSACHUSETTS
02208-2798
------------------------
The Prospectus is hereby amended and supplemented to add risk disclosure
regarding each Portfolio's investments in initial public offerings and to
reflect changes in the comparative indices for the Small Company Growth and
Technology Portfolios.
------------------------
The following paragraph is added to the "ADDITIONAL RISK FACTORS AND
INFORMATION" section on page 7:
INITIAL PUBLIC OFFERINGS
The Portfolios may purchase shares issued as part of, or a short period after, a
company's initial public offering ("IPOs"), and may at times dispose of those
shares shortly after their acquisition. A Portfolio's purchase of shares issued
in IPOs exposes it to the risks associated with companies that have little
operating history as public companies, as well as to the risks inherent in those
sectors of the market where these new issuers operate. The market for IPO
issuers has been volatile, and share prices of newly-public companies have
fluctuated significantly over short periods of time.
------------------------
<PAGE>
The Russell 2000 Growth Index is being added as the primary comparative index
for the Small Company Growth Portfolio. The Russell 2000 Index will remain as a
secondary comparative index. MSDW Investment Management believes that the
Russell 2000 Growth Index is more representative of the growth-oriented
companies in the Small Company Growth Portfolio. Accordingly, the chart on
page 3 is hereby deleted and replaced with the following:
AVERAGE ANNUAL TOTAL RETURN (FOR THE YEAR ENDED DECEMBER 31, 1999)
<TABLE>
<CAPTION>
CLASS A CLASS B
(COMMENCED (COMMENCED RUSSELL 2000 RUSSELL 2000
OPERATIONS ON OPERATIONS ON GROWTH INDEX* INDEX**
NOVEMBER 1, 1989) JANUARY 2, 1996) CLASS A CLASS B CLASS A
<S> <C> <C> <C> <C> <C>
------------------------------------------------------------------------------------------------------------------------
PAST ONE YEAR 96.45% 95.97% 43.09% 43.09% 21.26%
------------------------------------------------------------------------------------------------------------------------
PAST FIVE YEARS 31.00% N/A 18.99% N/A 16.69%
------------------------------------------------------------------------------------------------------------------------
PAST TEN YEARS 19.43% N/A 13.51% N/A 13.40%
------------------------------------------------------------------------------------------------------------------------
SINCE INCEPTION 19.78% 30.09% 13.52%*** 16.78%*** 13.26%
<CAPTION>
RUSSELL 2000
INDEX**
CLASS B
<S> <C>
---------------------
PAST ONE YEAR 21.26%
---------------------
PAST FIVE YEARS N/A
---------------------
PAST TEN YEARS N/A
---------------------
SINCE INCEPTION 13.87%
</TABLE>
THE BAR CHART AND TABLE ABOVE SHOW THE PERFORMANCE OF THE PORTFOLIO YEAR-BY-YEAR
AND AS AN AVERAGE OVER DIFFERENT PERIODS OF TIME. TOGETHER, THE BAR CHART AND
TABLE DEMONSTRATE THE VARIABILITY OF PERFORMANCE OVER TIME AND PROVIDE AN
INDICATION OF THE RISKS OF INVESTING IN THE PORTFOLIO. IN ADDITION, THE TABLE
COMPARES THE PERFORMANCE OF THE PORTFOLIO TO INDICES OF SIMILAR SECURITIES. AN
INDEX IS A HYPOTHETICAL MEASURE OF PERFORMANCE BASED ON THE UPS AND DOWNS OF
SECURITIES THAT MAKE UP A PARTICULAR MARKET. THE INDICES DO NOT SHOW ACTUAL
INVESTMENT RETURNS OR REFLECT PAYMENT OF MANAGEMENT OR BROKERAGE FEES, WHICH
WOULD LOWER THE INDICES' PERFORMANCE. THE TABLE SEPARATELY COMPARES CLASS A AND
CLASS B SHARES TO EACH INDEX SO THAT YOU CAN COMPARE PERFORMANCE FOR EACH CLASS
FROM ITS COMMENCEMENT DATE. THE INDICES THEMSELVES DO NOT ACTUALLY HAVE TWO
CLASSES. HOW THE PORTFOLIO HAS PERFORMED IN THE PAST DOES NOT NECESSARILY
INDICATE HOW THE PORTFOLIO WILL PERFORM IN THE FUTURE.
*THE RUSSELL 2000 GROWTH INDEX IS A MARKET CAPITALIZATION-WEIGHTED INDEX
COMPRISED OF THOSE COMPANIES OF THE RUSSELL 2000 INDEX WITH HIGHER FORECASTED
GROWTH VALUES AND HIGHER PRICE-TO-BOOK RATIOS.
**THE RUSSELL 2000 INDEX IS AN UNMANAGED INDEX COMPRISED OF 2000 OF THE
SMALLEST COMPANIES IN THE RUSSELL 3000 INDEX AND REPRESENTS APPROXIMATELY 11%
OF THE APPROXIMATELY 98% OF THE INVESTIBLE U.S. EQUITY MARKET.
***THE INCEPTION DATES FOR THE RUSSELL 2000 GROWTH INDEX FOR CLASS A AND
CLASS B SHARES ARE NOVEMBER 30, 1989 AND JANUARY 31, 1996, RESPECTIVELY.
----------------------------------
The Nasdaq Composite Index is being added as the primary comparative index for
the Technology Portfolio. The S&P 500 Index will remain as a secondary
comparative index. MSDW Investment Management believes that the Nasdaq Composite
Index, which is one of the most widely followed and quoted major market indices
and includes over 5,000 companies, is more representative of the companies in
the Technology Portfolio. Accordingly, the chart on page 5 is hereby deleted and
replaced with the following:
AVERAGE ANNUAL TOTAL RETURN (FOR THE YEAR ENDED DECEMBER 31, 1999)
<TABLE>
<CAPTION>
CLASS A CLASS B
(COMMENCED (COMMENCED
OPERATIONS ON OPERATIONS ON NASDAQ COMPOSITE
SEPTEMBER 16, 1996) SEPTEMBER 16, 1996) INDEX* S&P 500 INDEX**
<S> <C> <C> <C> <C>
--------------------------------------------------------------------------------------------------------------
PAST ONE YEAR 160.62% 160.26% 85.59% 21.04%
--------------------------------------------------------------------------------------------------------------
SINCE INCEPTION 71.47% 71.13% 45.16% 28.15%
</TABLE>
THE BAR CHART AND TABLE ABOVE SHOW THE PERFORMANCE OF THE PORTFOLIO YEAR-BY-YEAR
AND AS AN AVERAGE OVER DIFFERENT PERIODS OF TIME. TOGETHER, THE BAR CHART AND
TABLE DEMONSTRATE THE VARIABILITY OF PERFORMANCE OVER TIME AND PROVIDE AN
INDICATION OF THE RISKS OF INVESTING IN THE PORTFOLIO. IN ADDITION, THE TABLE
COMPARES THE PERFORMANCE OF THE PORTFOLIO TO INDICES OF SIMILAR SECURITIES. AN
INDEX IS A HYPOTHETICAL MEASURE OF PERFORMANCE BASED ON THE UPS AND DOWNS OF
SECURITIES THAT MAKE UP A PARTICULAR MARKET. THE INDICES DO NOT SHOW ACTUAL
INVESTMENT RETURNS OR REFLECT PAYMENT OF MANAGEMENT OR BROKERAGE FEES, WHICH
WOULD LOWER THE INDICES' PERFORMANCE. HOW THE PORTFOLIO HAS PERFORMED IN THE
PAST DOES NOT NECESSARILY INDICATE HOW THE PORTFOLIO WILL PERFORM IN THE FUTURE.
*THE NASDAQ COMPOSITE INDEX IS A MARKET CAPITALIZATION-WEIGHTED INDEX COMPRISED
OF ALL COMMON STOCKS LISTED ON THE NASDAQ STOCK MARKET.
**THE S&P 500 INDEX IS COMPRISED OF THE STOCKS OF 500 LARGE-CAP U.S. COMPANIES
WITH MARKET CAPITALIZATION OF $1 BILLION OR MORE. THESE 500 COMPANIES
REPRESENT APPROXIMATELY 100 INDUSTRIES, CHOSEN MAINLY FOR MARKET SIZE,
LIQUIDITY AND INDUSTRY GROUP REPRESENTATION.
----------------------------------
PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE
