IL-1120-X: Amended Corporation Income and Replacement Tax Return (2025)
Illinois · Illinois DOR · Used for tax years 2025
Form IL-1120-X is Illinois’s Amended Corporation Income and Replacement Tax Return for tax years ending on or after December 31, 2025. It compares previously reported or adjusted amounts with corrected income, deductions, credits, and taxes, and calculates any refund or additiona
Who files this form
Form IL-1120-X is for corporations amending an Illinois Corporation Income and Replacement Tax Return. The form includes options for state changes, federal changes, and net loss deduction changes.
The 2025 version is for tax years ending on or after December 31, 2025. Do not use it for tax years ending before that date.
The form also includes provisions for unitary filers filing a combined return and foreign insurers.
What you'll need
Have your corporation’s legal name, mailing address, FEIN, NAICS code, Illinois corporate file number, and the beginning and ending dates of the tax year being amended ready. You will need the amounts most recently reported or adjusted and the corrected amounts.
The attachment checklist on page 4 lists documents to include:
- Federal Form 1120-X, plus federal Form 1139 or a federal RAR, if applicable.
- Federal finalization if Line 1 changed, such as a federal refund check copy, IRS audit report, or federal transcript verifying federal taxable income.
- Any and all out-of-state returns if Lines 28 through 30 changed.
- Schedules supporting the items you report, such as Form IL-4562 for special depreciation, Schedule 80/20 for related-party expenses, Schedule M for other additions or subtractions, and Schedule NB for nonbusiness income.
- Schedule NLD or UB/NLD for an Illinois net loss deduction, and federal Form 982 for a discharge of indebtedness adjustment.
- Credit and recapture forms, as applicable: Schedule 4255, Form IL-477, and Schedule 1299-D, with required support.
- Schedules K-1-P or K-1-T for applicable income, adjustments, pass-through withholding, or pass-through entity tax credits. For Illinois income tax withholding on Line 59e, attach copies of all Forms W-2G.
Special filings may require additional attachments, including unitary schedules, foreign insurer schedules, Schedule INL, or Form IL-2220. Use the page 4 checklist to match your completed lines to the required documents.
How to file
- Identify your corporation and the tax year you are amending. Check the applicable type of change in Step 1.
- Explain the changes in the space provided. Attach a separate sheet if necessary.
- Enter the amounts most recently reported or adjusted in Column A and the corrected amounts in Column B. Complete the applicable income, tax, payment, and refund calculations.
- Attach the required supporting documents. Have an authorized officer sign and date the return and provide their title and phone number.
- If you owe tax on Line 68, complete Form IL-1120-X-V. Make your check or money order payable to “Illinois Department of Revenue” and write your FEIN, tax year ending, and “IL-1120-X-V” on it. Attach the voucher and payment to page 1, and enter the payment amount at the top of that page.
Mail the return to:
Illinois Department of Revenue
P.O. Box 19016
Springfield, IL 62794-9016
Common mistakes
- Using the wrong version. This version cannot be used for tax years ending before December 31, 2025.
- Leaving out the explanation of changes. The form specifically directs you to explain the changes and attach a separate sheet if needed.
- Completing Step 4 when it does not apply. If Line 23 is derived only inside Illinois and you are not a unitary filer, check box A above Step 4 and leave Lines 24 through 34 blank.
- Leaving Step 4 incomplete when it applies. If any portion of Line 23 is derived outside Illinois, or you are a unitary filer, check box B and complete all Step 4 lines. Do not leave Lines 28 through 30 blank.
- Omitting required attachments. The form warns that missing documents may cause the corresponding line item to be disallowed.
- Omitting required information. The form warns that failure to provide required information could result in a penalty.
Questions people ask
Can I file this form only to report an increased net loss?
Yes. Check Step 1, box D if you are filing only to report an increased net loss on Line 37, Column B.
What must I include for a federal change?
In Step 1, item P, select “Federal change,” indicate whether it is “Partial agreed” or “Finalized,” enter the finalization date, and attach your federal finalization.
Can I apply an overpayment to a future year instead of receiving it all as a refund?
Line 66 lets you enter the portion of the Line 65 overpayment to be credited forward. Line 67 shows the remaining refund. If the carryforward is going to a different FEIN, check the box at Line 66 and attach a detailed statement.
Will I be billed for additional penalties and interest?
Yes. The form states that you will be sent a bill for any additional penalty and interest due.