IRS form used to amend a previously filed individual income-tax return. For example, after you receive a CP2000 notice from the IRS for a tax underpayment, you may need to file an amended tax return on Form 1040-X (see related FAQs on stock options and restricted stock/RSUs).

You generally have three years from the time you filed the original return or two years from when you made any payment (whichever is later) to submit Form 1040-X to correct mistakes or add missing information. If you submit Form 1040X for bad debt or a worthless security, you have seven years after the due date of the return that you filed for the tax year in which the debt or security became worthless.

Note that Form 1040-X can now be filed electronically with the IRS.