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Exhibit 15

June 24, 2003

Board of Directors and Stockholders
Aon Corporation

We are aware of the incorporation by reference in the Registration Statement (Post-Effective Amendment No. 2 to Form S-3 No. 333-102799) and related Prospectus of Aon Corporation of our report dated May 9, 2003 relating to the unaudited condensed consolidated interim financial statements of Aon Corporation which is included in its Form 10-Q for the quarter ended March 31, 2003.

/s/ Ernst & Young LLP

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Exhibit 15