<PAGE>   1
                          SHORT-TERM INVESTMENTS TRUST

                               TREASURY PORTFOLIO

                            (CASH MANAGEMENT CLASS)
                             (INSTITUTIONAL CLASS)
                          (PERSONAL INVESTMENT CLASS)
                           (PRIVATE INVESTMENT CLASS)
                                (RESOURCE 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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