Exhibit 99(a)

CERTIFICATION OF PERIODIC REPORT

I, Dennis J. Kuester, Chief Executive Officer of Marshall & Ilsley Corporation (the “Company”), certify, pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, 18 U.S.C. Section 1350, that:

(1) the Annual Report on Form 10-K of the Company for the fiscal year ended December 31, 2002 (the “Report”) fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934 (14 U.S.C. 78m or 78o(d)); and

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

Dated: March 10, 2003.

/s/ Dennis J. Kuester

Dennis J. Kuester

Chief Executive Officer

Marshall & Ilsley Corporation