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How far back can I file an amended return to claim a refund based on a state change only?

If your change decreases the tax due to Illinois and you want a refund, you must file an amended return (claim for overpayment) within three years after the extended due date, three years after the date your original return was filed, or one year after the date your Illinois tax was paid, whichever is latest.

Note: The refund allowed on a claim filed within three years after the original return was filed is limited to the total of all payments made within three years and six months prior to the filing of the refund claim. If your change increases the tax you owe to Illinois, you should file a Form IL-1040-X and pay the tax, penalty, and interest as soon as you realize that it is owed.

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