EXHIBIT 99.1



                              [Company Letterhead]


                  Certification of Principal Executive Officer
                           Pursuant to 18 U.S.C. 1350
                 (Section 906 of the Sarbanes-Oxley Act of 2002)

I, Mario R. Houthooft, President and Chief Executive Officer (principal
executive officer) of VASCO Data Security International, Inc. (the
"Registrant"), certifies that to the best of my knowledge, based upon a review
of the Quarterly Report on Form 10-Q for the period ended June 30, 2002 of the
Registrant (the "Report"):

(1)  The Report fully complies with the requirements of Section 13(a) of the
     Securities Exchange Act of 1934, as amended; and

(2)  The information contained in the Report fairly presents, in all material
     respects, the financial condition and results of operations of the
     Registrant.


/s/ Mario R. Houthooft
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Name: Mario R. Houthooft
Date: August 14, 2002