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                                                                    Exhibit 99.1

              CERTIFICATION PURSUANT TO SECTION 1350 OF CHAPTER 63
                      OF TITLE 18 OF THE UNITED STATES CODE

     I, Patrick G. Ryan, the Chief Executive Officer of Aon Corporation (the
"COMPANY"), certify that (i) the Annual Report on Form 10-K of the Company for
the year ended December 31, 2002 (the "REPORT") fully complies with the
requirements of Section 13(a) or 15(d), as applicable, of the Securities
Exchange Act of 1934, as amended; and (ii) the information contained in the
Report fairly presents, in all material respects, the financial condition and
results of operations of the Company.


                                           /s/ Patrick G. Ryan
                                           ---------------------------
                                           Patrick G. Ryan
                                           Chief Executive Officer
                                           March 26, 2003