IL-1120-ST-X: Amended Small Business Corporation Replacement Tax Return (2025)
Illinois · Illinois DOR · Used for tax years 2025
Form IL-1120-ST-X is Illinois’s Amended Small Business Corporation Replacement Tax Return. It reports corrected income, deductions, taxes, credits, and payments to determine a refund or additional tax due, including any applicable pass-through entity tax and pass-through withhold
Who files this form
Form IL-1120-ST-X is for small business corporations amending an Illinois Small Business Corporation Replacement Tax Return.
The 2025 version is for tax years ending on or after December 31, 2025. You may not use it to amend a return for a tax year ending before that date.
The form includes boxes for a net loss deduction (NLD) change, a state change, or a federal change. It also has a box for returns filed only to report an increased net loss on Line 49, Column B.
What you'll need
Have the amounts most recently reported or adjusted and the corrected amounts ready. The form also asks for your legal business name, mailing address, FEIN, NAICS code, and Illinois corporate file number.
Attach supporting documents that apply to your changes, including:
- Your federal finalization for a federal change.
- Schedule UB if you are a member of a unitary business group.
- Form IL-4562 for Illinois special depreciation, Schedule 80/20 for related-party expenses, and Schedule M for other additions or subtractions.
- Schedule 1299-A for the subtractions listed on Lines 25 through 29.
- Schedules K-1-P or K-1-T for distributive share adjustments or pass-through withholding reported to you.
- Schedule B for the shareholder amounts and pass-through withholding identified on the return.
- Schedule NB for nonbusiness income or loss, Schedule NLD for an Illinois net loss deduction, and federal Form 982 for a discharge of indebtedness adjustment.
- Schedule 4255 for recaptured investment credits and Form IL-477 for investment credits.
- Forms W-2G for Illinois income tax withholding.
- Form IL-2220 if you are paying PTE tax and annualized your income on that form.
If you completed federal Form 8886 or federal Schedule M-3, Part II, Line 10, attach the federal forms if you have not already done so. Attach a detailed statement for merged losses or an overpayment carryforward going to a different FEIN.
How to file
- Enter the beginning and ending dates of the tax year you are amending and complete the business identification information.
- Check the applicable change boxes and explain the changes. Attach a separate sheet if needed.
- 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 or balance-due steps.
- Attach the required supporting forms and schedules. If paying with the return, enter the payment amount at the top of Page 1 and attach your payment and Form IL-1120-ST-X-V where indicated.
- Have an authorized officer sign and date the return and provide their title and phone number.
Mail the return to:
Illinois Department of Revenue
P.O. Box 19016
Springfield, IL 62794-9016
Common mistakes
- Using the wrong version. Do not use the 2025 form for a tax year ending before December 31, 2025.
- Leaving out the explanation of changes. The form specifically asks you to explain the changes and allows a separate sheet if needed.
- Completing Step 6 incorrectly. If Line 35 income is derived only inside Illinois and you are not a unitary filer, check Line A and leave Lines 36 through 46 blank. If any portion is derived outside Illinois, or you are a unitary filer, check Line B and complete all of Step 6. Do not leave Lines 40 through 42 blank.
- Entering negative sales amounts. Lines 40 and 41 cannot be negative.
- Forgetting the payment entry. Enter the amount paid with the return at the top of Page 1.
- Completing Schedule B out of order. Read its instructions and complete the required K-1 schedules and Section B before Section A. The form warns that failure to follow these instructions may delay processing or lead to requests for more information.
Questions people ask
What goes in Columns A and B?
Column A shows the amounts most recently reported or adjusted. Column B shows the corrected amounts.
What if I am amending only to report a larger net loss?
Check the box in Step 1, Item C, if you are filing only to report an increased net loss on Line 49, Column B.
What must I include for a federal change?
Check the federal change box and indicate whether the change is partially agreed or finalized. The form asks for the finalization date and requires you to attach your federal finalization.
Can an overpayment be credited forward instead of refunded?
Line 70 lets you enter the amount of the Line 69 overpayment to credit forward. Line 71 is the remaining amount to be refunded. If the carryforward is going to a different FEIN, check the box and attach a detailed statement.
How will I know about additional penalty and interest?
The form says you will be sent a bill for any additional penalty and interest.