Cost basis, Form 3922 and Form 3921
These forms are not filed with your return, but without them your basis calculations are guesswork.
Form 3921
Issued for ISO exercises. It reports the exercise date, strike price and fair market value at exercise — the inputs to your AMT adjustment and dual basis.
Form 3922
Issued for ESPP purchases. It reports the offering date, purchase date, purchase price and FMV — the inputs to the bargain element.
Keep them forever
Holding periods run for years. Store both forms with your permanent records, not just with the year's return.
This article is educational and not tax, legal, or financial advice. Talk to a CPA or tax advisor about your specific situation.