Why ISOs and the AMT collide
When you exercise an incentive stock option (ISO), you pay no regular income tax, even if the shares are worth far more than your strike price. But the Alternative Minimum Tax — a parallel tax system with fewer deductions — does count the gain. The difference between the fair market value at exercise and your strike, the bargain element, is added to your AMT income the year you exercise and hold.
The paper-gain trap
The danger is that AMT can be owed in real cash on a gain you have not realized. If you exercise, hold past year-end, and the stock later falls, you can still owe AMT on the original bargain element — a bill that in a crash can exceed what the shares are now worth. This is why people who exercised at a high valuation and held through a downturn were hit hardest. Keeping the bargain element small, or selling enough shares to cover the AMT, is how you manage it.
The bargain element
The bargain element is simply (fair market value at exercise − strike price) × number of options. On its own it is not taxed for regular purposes at exercise, but for AMT it is treated as income. The larger the spread between the FMV and your strike, the larger the AMT exposure — which is why exercising early, when the FMV is close to the strike, keeps AMT low and starts the holding-period clock.
The AMT credit
AMT you pay because of an ISO exercise generally becomes a minimum-tax credit you can use in future years when your regular tax exceeds your AMT. Over time it is often recovered, so the AMT can be a timing cost rather than a permanent one. This is why you must track two cost bases: your regular-tax basis (the strike) and your AMT basis (the higher FMV). The difference drives the credit when you eventually sell.
Qualifying vs disqualifying
Sell the shares more than two years after grant and more than one year after exercise for a qualifying disposition: the entire gain over the strike is a long-term capital gain, the best outcome. Sell sooner and it is disqualifying: the bargain element becomes ordinary income and there is no AMT preference, which is sometimes used deliberately to avoid AMT by exercising and selling in the same year.
A quick example
You exercise 1,000 ISOs at a $2 strike when the FMV is $12. The bargain element is $10,000. Exercise and hold, and roughly $2,600 of AMT may apply that year even though you sold nothing. Hold to a qualifying disposition and sell at $20: the entire $18,000 gain over the strike is long-term, and the AMT you paid becomes a credit. Exercise and sell in the same year and instead $10,000 is ordinary income with no AMT.