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.

If you have additional inquiries, you may submit them to the Questions, Comments, or Request form.