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Illinois Department of Revenue Offers Guidance on Amending State Tax Returns

Press Release - Tuesday, June 03, 2025

CHICAGO - The Illinois Department of Revenue (IDOR) is reminding taxpayers that if they discover an error on a previously filed state income tax return—such as a miscalculation, a missed deduction, or unreported income—they are required to file an amended return to correct the mistake.

"It's not uncommon for taxpayers to spot an error after they've filed their returns," said IDOR Director David Harris. "Amending a return is a straightforward way to make it right and ensures taxpayers are paying exactly what they owe—or getting the refunds they're due."

How to File an Amended Return:
  1. Use the Correct Form - Always file Form IL-1040-X to amend an individual income tax return.
  2. Include Supporting Documents - If you are amending due to a federal adjustment, wait until the Internal Revenue Service (IRS) finalizes it, and include supporting proof with your amended return.
  3. Provide a Clear Explanation - Use Step 11, Line D of Form IL-1040-X to clearly explain the reason for the amendment. Incomplete or unclear explanations may delay processing.
  4. Sign and Date the Form - Both taxpayers must sign and date the form if filing jointly.
  5. Submit the Completed Return - Taxpayers can file electronically through the modernized-file system (MeF) starting with the 2021 tax year and may also send completed forms and documents by mail.
Note: Taxpayers do not need to file an amended return for changes to names, addresses, or Social Security numbers. These updates can be made through MyTax Illinois or by mail. 

IDOR encourages taxpayers to file as soon as an error is identified.

"If a taxpayer finds a mistake, it's best to correct it without delay," Director Harris said. "Promptly filing an amended return will help avoid unnecessary penalties and interest if additional tax is owed—and if a refund is due, it will result in getting the money back faster."

For complete instructions, taxpayers can refer to the Form IL-1040-X instructions, available on the IDOR website. Additional help is available by visiting tax.illinois.gov or calling 1-800-732-8866 or 217-782-3336.