Form IL-1040-X: Amended Individual Income Tax Return

The Illinois Amended Individual Income Tax Return (Form IL 1040X) allows you to make changes to your previously filed Illinois income tax return for a specific tax year. This is necessary if you discover errors, need to report additional income, or claim missed deductions or credits after filing your initial return.

When to File Form IL 1040X?

A. Federal Changes: If the IRS amends your federal return (Form 1040X) and it affects your Illinois taxes, you may need to file Form IL 1040X.

B. Errors: You discovered errors on your original return, like miscalculation of income, deductions, or credits.

C. Missed Information: You need to report additional income or claim missed deductions and credits you weren't aware of when filing initially.

How to File Form IL 1040X?

You can file Form IL 1040X electronically through IDOR's website: or mail a paper copy to the address provided on the form.

Key Points to Remember About Form IL 1040X:

A. File a separate Form IL 1040X for each tax year you want to amend.

B. Do not use Form IL 1040X for name, address, or Social Security number changes. Notify the Illinois Department of Revenue (IDOR) directly.

C. Filing deadlines vary depending on the reason for amendment:

  • Federal changes:
  • To claim a refund (overpayment): Within two years plus 120 days of the federal finalization date.
  • To pay additional tax: Within 120 days of the federal finalization date.
  • Other amendments: Generally, within three years from the original due date of the return (including extensions).