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1 Filed Pursuant To Rule 424(b)(3) Registration No. 333-63439 Prospectus Supplement No. 1 To Prospectus Dated December 10, 1998 $86,250,000 Aspen Technology, Inc. 5-1/4% Convertible Subordinated Debentures due June 15, 2005 This Prospectus Supplement supplements the information we provided in our December 10, 1998 Prospectus relating to our 5-1/4% Convertible Subordinated Debentures due June 15, 2005 and the shares of our common stock that we must issue upon conversion of those Debentures. This Prospectus Supplement is not complete without the Prospectus, and we have not authorized anyone to deliver or use this Prospectus Supplement without the Prospectus. In this Prospectus Supplement, we use some capitalized terms that we defined in the Prospectus. You should read the Prospectus to understand those terms. Unless we indicate otherwise, the information in this Prospectus Supplement is as of January 21, 1999. We are providing this Prospectus Supplement to add the following information to the table in the Prospectus under the caption "Selling Securityholders": - ------------ * Less than one percent. (1) For purposes of this Prospectus Supplement only, we have calculated this percentage on the basis of $86,250,000 aggregate principal amount of Debentures outstanding on January 21, 1999. (2) Each Selling Securityholder has indicated that it intends to offer all of the Debentures that it owns. As a result, no Debentures will be owned after the offering. (3) For purposes of this Prospectus Supplement only, we have calculated this number using the conversion rate described on the front cover page of the Prospectus. (4) For purposes of this Prospectus Supplement only, we have calculated this percentage on the basis of 24,926,493 shares of common stock outstanding on January 21, 1999. The date of this Prospectus Supplement is January 25, 1999.
Principal Amount of Shares of Common Shares of Common Debentures Beneficially Stock Beneficially Stock Beneficially Owned Prior to the Owned Prior to the Shares of Owned After the Offering Amount of Offering Common Offering ----------------------- Debentures -------------------- Stock ------------------- Selling Securityholder Number Percent(1) Offered(2) Number(3) Percent(4) Offered(3) Number Percent(4) ---------- ----------- ---------- --------- ---------- ---------- ------ ---------- Goldman, Sachs & Co. $1,539,000 1.8% $1,539,000 29,054 * 29,054 -- -- Reserve Convertible 650,000 * 650,000 12,271 * 12,271 -- -- Securities Fund McMahan Securities 48,000 * 48,000 906 * 906 -- -- Company, L.P.