Form 3921: the return a company files for every ISO exercise

What the form is for

An incentive stock option produces no regular-tax income when it is exercised. That is precisely why the IRS needs a return: without one, an event with real tax consequences would leave no trace until the shares are eventually sold, possibly years later and possibly by someone who no longer works at the company.

Form 3921 fills that gap. It records the facts needed to compute two later things: the alternative minimum tax adjustment in the year of exercise, and the character and amount of gain when the shares are sold. It is an information return in the same family as the 1099 series — the corporation reports, the individual uses the information, and the IRS holds a matching copy.

Who has to file

The instructions put the obligation on the corporation: every corporation which in any calendar year transfers to any person a share of stock pursuant to that person’s exercise of an incentive stock option must file Form 3921 for that year. The trigger is the transfer of the share, not the grant of the option and not the vesting of it.

Private companies are not excused. The obligation applies whether or not there is a market for the shares, whether or not the option holder is still employed, and whether or not any cash changed hands beyond the exercise price. If the corporation transferring the stock is not the corporation that granted the option, box 6 carries the name, address, and taxpayer identification number of the corporation whose stock is being transferred.

What each box reports

Boxes 3, 4 and 5 are the ones that produce the number people care about. Fair market value at exercise minus exercise price, multiplied by shares, is the bargain element that enters the alternative minimum tax calculation for the year of exercise. That mechanism is set out in the guide to AMT and incentive stock options.

The deadlines

The electronic filing threshold is the part that has changed most recently. Since 1 January 2024 filers must submit information returns electronically if they are filing 10 or more returns in aggregate across all information return types — not 10 of each type. A company issuing a handful of 1099s and a handful of Form 3921s can cross the threshold without noticing.

Penalties for failing to file a correct information return, or failing to furnish a correct statement, sit in sections 6721 and 6722. The amounts are adjusted for inflation and vary with how late the correction is and the size of the filer, so the current figures belong in the General Instructions for Certain Information Returns rather than in an article. The practical point is that they are per return, and a company with a hundred exercises has a hundred returns.

What the employee does with their copy

Nothing is attached to the return. The copy is source data. In the year of exercise it supplies the two per-share figures needed for the alternative minimum tax adjustment. Later, when the shares are sold, it supplies the grant and exercise dates that determine whether the section 422(a)(1) holding period was met, and the price information that feeds the basis calculation.

That "later" can be a very long time. Options exercised in a private company are frequently sold five or ten years afterwards, in an acquisition, by someone who has changed jobs twice and email addresses three times. The single most useful thing to do with a Form 3921 is to file it somewhere that will still exist then, alongside the grant agreement and any 83(b) election.

If a form never arrives, the underlying facts still exist in the company’s records — the option agreement has the grant date and strike, and the exercise paperwork has the date and the board’s fair market value determination. Ask the company for a copy rather than reconstructing from memory. A missing statement is the company’s compliance problem, but it becomes the individual’s evidence problem at sale.

Form 3921 versus Form 3922

The two forms share one set of instructions and are easy to confuse. Form 3921 covers the exercise of an incentive stock option. Form 3922 covers the first transfer of legal title to a share acquired under an employee stock purchase plan where the option had a discounted exercise price. A company running only an option plan will never file a 3922; a company running both will file both, on the same calendar.

Neither form reports non-statutory options. An NSO exercise is compensation, so it runs through payroll and appears on Form W-2 for employees, or on the appropriate information return for non-employees. If a company thinks it is issuing ISOs but the awards fail a section 422 condition — the $100,000 annual limit in section 422(d) is a common one — the reporting route changes with them.