First Midwest Bancorp (ONBPP) 10-Q 2015 Q1 Quarterly report

Exhibit 15

Acknowledgement of Independent Registered Public Accounting Firm

May 4, 2015

The Board of Directors of First Midwest Bancorp, Inc.

We are aware of the incorporation by reference in the following Registration Statements of First Midwest Bancorp, Inc. (the “Company”) of our report dated May 4, 2015, relating to the unaudited consolidated interim financial statements of the Company that are included in its Form 10-Q for the quarter ended March 31, 2015.

• Registration Statement (Form S-3 No. 33-20439) pertaining to the First Midwest Bancorp, Inc. Dividend Reinvestment and Stock Purchase Plan;

• Registration Statement (Form S-4 No. 333-114406) pertaining to First Midwest Capital Trust I;

• Registration Statement (Form S-4 No. 333-198561) pertaining to the registration of equity securities in connection with the acquisition of Great Lakes Financial Resources, Inc.;

• Registration Statement (Form S-8 No. 33-25136) pertaining to the First Midwest Bancorp, Inc. Savings and Profit Sharing Plan;

• Registration Statement (Form S-8 No. 33-42980) pertaining to the First Midwest Bancorp, Inc. 1989 Omnibus Stock and Incentive Plan;

• Registration Statement (Form S-8 No. 333-42273) pertaining to the First Midwest Bancorp, Inc. 1989 Omnibus Stock and Incentive Plan;

• Registration Statement (Form S-8 No. 333-61090) pertaining to the First Midwest Bancorp, Inc. 1989 Omnibus Stock and Incentive Plan;

• Registration Statement (Form S-8 No. 333-159389) pertaining to the First Midwest Bancorp, Inc. 1989 Omnibus Stock and Incentive Plan;

• Registration Statement (Form S-8 No. 333-168973) pertaining to the First Midwest Bancorp, Inc. Amended and Restated Omnibus Stock and Incentive Plan;

• Registration Statement (Form S-8 No. 333-63097) pertaining to the First Midwest Bancorp, Inc. Nonqualified Retirement Plan;

• Registration Statement (Form S-8 No. 333-63095) pertaining to the First Midwest Bancorp, Inc. Non-employee Directors’ Stock Option Plan;

• Registration Statement (Form S-8 No. 333-50140) pertaining to the First Midwest Bancorp, Inc. Non-employee Directors’ 1997 Stock Option Plan; and

• Registration Statement (Form S-8 No. 333-151072) pertaining to the First Midwest Bancorp, Inc. Amended and Restated Non-employee Directors Stock Plan.

/s/ ERNST & YOUNG LLP

Chicago, IL