<PAGE>   1

                          SHORT-TERM INVESTMENTS TRUST

                        TREASURY TAXADVANTAGE PORTFOLIO

                             (INSTITUTIONAL CLASS)
                           (PRIVATE INVESTMENT CLASS)

                        Supplement dated October 1, 1998
       to the Statement of Additional Information dated December 17, 1997


         The following information should be inserted within the discussion
under the heading  "MANAGEMENT--Trustees and Officers":

<TABLE>
 <S>                                      <C>                    <C>
 "PREMA MATHAI-DAVIS (48)                 Trustee                Chief Executive Officer, YWCA of
 350 Fifth Avenue, Suite 301                                     the U.S.A.; Commissioner, New
 New York, NY 10118                                              York City Department for
                                                                 the Aging; and  Member of the
                                                                 Board of Directors, Metropolitan
                                                                 Transportation Authority of New
                                                                 York State.
                                          
 EDWARD K. DUNN, JR. (63)                 Trustee                Chairman of the Board of
 2 Hopkins Plaza, 20th Floor                                     Directors, Mercantile Mortgage
 Baltimore, MD 21201                                             Corp; Formerly, Vice Chairman of
                                                                 the Board of Directors and
                                                                 President, Mercantile - Safe
                                                                 Deposit & Trust Co.; and
                                                                 President, Mercantile
                                                                 Bankshares.
</TABLE>

Mr. Kroeger resigned as a Trustee of the Trust on June 11, 1998 and on that
date became a consultant to the Trust."



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