Guides
Form 3921: Reporting Incentive Stock Option (ISO) Exercises
Form 3921, “Exercise of an Incentive Stock Option Under Section 422(b),” is the IRS information return a corporation files for each employee who exercises an incentive stock option (ISO) during the calendar year. It records the grant date, exercise date, exercise price, and fair market value at exercise. The form itself reports no income, but it supplies the numbers you need for the alternative minimum tax (AMT) and for tracking your holding periods.
You do not attach Form 3921 to your Form 1040. It is a reference document. The company files Copy A with the IRS and sends Copy B to you, and you keep it to calculate any AMT adjustment in the exercise year and your cost basis when you eventually sell.
What Form 3921 is and who files it
Form 3921 is an information return that a corporation must file for each transfer of stock to a person who exercised an ISO described in IRC Section 422(b). The company files one form per exercise per employee. It is issued by employers, not by employees, and it carries no tax due on its own.
The filing obligation sits with the corporation whose stock was transferred. Companies that grant ISOs to founders, early employees, and staff generate one Form 3921 for every ISO exercise in a year. A single employee who exercises on three separate dates receives three forms.
The purpose is data matching. The IRS uses Form 3921 to track the AMT preference created at exercise and to confirm that gains reported at sale line up with the correct cost basis and holding periods. That is why the form matters even in a year when you owe nothing extra.
Filing deadlines for the corporation
The corporation must furnish Copy B to the employee by January 31 of the year after exercise (the IRS lists the recipient statement deadline; for the 2025 exercise year the furnish date fell in early February when January 31 landed on a weekend). Copy A goes to the IRS by February 28 if filing on paper, or March 31 if filing electronically. Businesses filing 10 or more information returns in aggregate must file electronically under current IRS rules.
Late or incorrect filings can trigger penalties under IRC Section 6721 and 6722, which scale by how late the return is and by business size. The exact per-form penalty amounts are indexed annually, so a company should confirm the current figures before a late filing.
The Form 3921 boxes, line by line
Form 3921 has six numbered boxes plus transferor and employee identification fields. Each box feeds a specific tax calculation: two boxes set your holding-period clocks, two set the AMT bargain element, and one sets the share count that scales everything.
| Box | Label | What it drives |
|---|---|---|
| Box 1 | Date option granted | Starts the 2-year-from-grant holding period |
| Box 2 | Date option exercised | Starts the 1-year-from-exercise holding period |
| Box 3 | Exercise price per share | Your regular-tax cost basis per share |
| Box 4 | Fair market value (FMV) per share on exercise date | The higher value used for the AMT adjustment |
| Box 5 | Number of shares transferred | Multiplier for the bargain element and basis |
| Box 6 | Name, address, TIN of corporation whose stock was transferred (only if different from the transferor) | Identifies the issuer when it is not the filer |
Box 3 and Box 4 are the pair that create the AMT preference. The gap between them, times Box 5, is your bargain element. Box 1 and Box 2 are the pair you use to test whether a later sale is a qualifying disposition.
Keep every Form 3921 you receive for as long as you hold the shares, plus at least three years after you sell. The FMV in Box 4 becomes part of your AMT cost basis, and you may need it years after exercise. For more on tracking purchase price and adjustments, see our guide to cost basis and why it matters.
The bargain element as an AMT preference item
The bargain element is the fair market value at exercise (Box 4) minus the exercise price (Box 3), multiplied by the number of shares (Box 5). For a regular ISO exercise where you hold the shares, this amount is zero for regular tax but is added to income as a preference item for the alternative minimum tax.
Example: you exercise 10,000 ISOs at a $1 strike (Box 3) when the FMV is $11 (Box 4). The bargain element is ($11 minus $1) times 10,000, or $100,000. Nothing appears on your regular Form 1040 for this exercise. But $100,000 is added to your alternative minimum taxable income and reported on Form 6251.
Whether that preference actually costs you depends on your full AMT picture. AMT applies when your tentative minimum tax exceeds your regular tax. A large bargain element can push you over that line, so many people who exercise and hold owe AMT in the exercise year even though they received no cash. Our explainer on the alternative minimum tax and how it works in 2026 walks through the exemption amounts and the phaseout.
The AMT you pay on an ISO exercise is often recoverable. It generally creates a minimum tax credit that you can carry forward and use in later years when your regular tax exceeds your tentative minimum tax. The credit can take several years to fully absorb, depending on income.
Two cost bases: regular and AMT
An ISO exercise leaves you with two different cost bases in the same shares. Your regular-tax basis equals the exercise price (Box 3 times Box 5). Your AMT basis equals the fair market value at exercise (Box 4 times Box 5), because you already recognized the bargain element for AMT.
This dual basis matters at sale. Under regular tax, gain is measured from the lower exercise-price basis. Under AMT, gain is measured from the higher FMV basis, which produces a smaller AMT gain and helps unwind the earlier preference. You track both figures on Form 6251 in the sale year.
Holding-period rules and qualifying dispositions
A qualifying disposition requires you to hold the ISO shares for more than 2 years from the grant date (Box 1) and more than 1 year from the exercise date (Box 2). Meet both, and the entire gain over your exercise price is taxed as long-term capital gain under the regular system. Miss either, and the sale is a disqualifying disposition.
The two clocks run from different dates on the form, so you must satisfy both. Box 1 sets the grant-date clock and Box 2 sets the exercise-date clock. Because grant usually precedes exercise, the 2-year-from-grant test is often the binding one for shares exercised soon after grant.
| Disposition type | Holding periods met | Regular-tax treatment |
|---|---|---|
| Qualifying | More than 2 years from grant AND more than 1 year from exercise | Full gain over exercise price taxed as long-term capital gain |
| Disqualifying | Either period not met | Bargain element (or actual gain if less) taxed as ordinary income; remainder as capital gain |
In a disqualifying disposition, the bargain element, capped at the actual gain, becomes ordinary compensation income. Employers typically report this on your Form W-2. Any additional gain above that is capital gain, short-term or long-term depending on how long you held. A disqualifying disposition also effectively reverses the AMT preference for that exercise, since you no longer have the ISO tax benefit to protect.
If you sell in the same calendar year you exercised, the exercise never becomes an AMT preference item at all, because the disqualifying disposition converts it to regular-tax ordinary income. This same-year sale is a common way to cap AMT exposure when the stock has run up sharply.
How to use Form 3921 on your tax return
You do not file Form 3921 itself, but you use its numbers in up to three places: Form 6251 for the AMT adjustment in the exercise year, Form 8949 and Schedule D for gain or loss when you sell, and your W-2 reconciliation if the sale was disqualifying. Keep the form to support each figure.
In the exercise year, if you exercised and held, carry the Box 4 minus Box 3 spread times Box 5 to Form 6251 as an ISO adjustment. In the sale year, report the sale on Form 8949 and Schedule D, using your regular-tax basis, and compute a separate AMT gain using your FMV-based AMT basis on Form 6251.
Reconcile against your W-2 in a disqualifying-disposition year. The ordinary-income portion should already appear in your wages, so you adjust your Form 8949 basis upward by that amount to avoid double-taxing it. Getting this reconciliation right is where most ISO sale errors happen.
Frequently asked questions
Do I report Form 3921 on my tax return?
You do not attach Form 3921 to your return, and exercising an ISO alone creates no regular taxable income. You use the form’s numbers to compute an AMT adjustment on Form 6251 if you exercised and held the shares, and to establish your cost basis and holding periods for when you sell. Keep the form with your records.
What is the bargain element on Form 3921?
The bargain element is the fair market value per share at exercise (Box 4) minus the exercise price per share (Box 3), multiplied by the number of shares (Box 5). For an ISO you exercise and hold, this amount is a preference item for the alternative minimum tax and appears on Form 6251, even though it is not regular taxable income that year.
Does exercising an ISO trigger AMT?
Exercising and holding ISO shares can trigger the alternative minimum tax because the bargain element is added to alternative minimum taxable income. Whether you actually owe depends on your full return, since AMT applies only when your tentative minimum tax exceeds your regular tax. A large spread on a big exercise is a common AMT trigger, and the tax may create a credit you recover later.
What are the ISO holding-period rules?
For a qualifying disposition, you must hold the shares more than 2 years from the grant date in Box 1 and more than 1 year from the exercise date in Box 2. Meeting both means the full gain over your exercise price is long-term capital gain. Missing either creates a disqualifying disposition, where the bargain element is taxed as ordinary income.
What happens in a disqualifying disposition?
In a disqualifying disposition, you sold before meeting one or both holding periods. The bargain element, capped at your actual gain, becomes ordinary compensation income, usually reported on your W-2. Any gain above that is capital gain based on your holding period. The exercise-year AMT preference is effectively unwound, and you adjust your Form 8949 basis to avoid double taxation.
Why do I have two cost bases for the same shares?
An ISO exercise creates a regular-tax basis equal to your exercise price and an AMT basis equal to the fair market value at exercise (Box 4). You recognized the bargain element for AMT but not for regular tax, so the two systems measure gain from different starting points. At sale you compute regular gain and AMT gain separately on Form 6251.
Who has to file Form 3921?
The corporation whose stock was transferred files Form 3921, one per ISO exercise per employee, for each transfer under Section 422(b). Employers must furnish Copy B to the employee by January 31 and file Copy A with the IRS by February 28 on paper or March 31 electronically. Employees receive the form; they do not file it.
Reviewed by The Ledgerism Editorial Team. Last reviewed: July 2026.