<PAGE>   1

                          SHORT-TERM INVESTMENTS TRUST

                         GOVERNMENT & AGENCY PORTFOLIO

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

                       Supplement dated October 1, 1998
      to the Statement of Additional Information dated September 1, 1998


         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.
</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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