When you exercise and hold an Incentive Stock Option (ISO) there is no taxable event for ordinary tax. You merely purchased the stock, and your cost basis is the strike price. However, for AMT purposes, the difference between what you paid and what the stock is worth at the time of exercise is a "preference item" for AMT. What that means is that amount is added to your income for the AMT calculation. In your case, it was quite a bit, so it put you in a situation of having to pay AMT. That preference amount carries forward and when you sell the stock you will get an AMT credit. However, while the amount that can be added to AMT income from exercising ISOs is unlimited, the amount of the credit you can take is limited each year. So, you may or may not recover the full amount of AMT tax you paid upon exercise in the year you sell (if you sell it all in the same year). Unused preference amounts do roll forward and you will eventually recapture all of it (more than likely).
The calculations are complicated and depend upon your overall tax situation at the time, so it's impossible to say with any specificity what yours will look like. Make sure you keep copies all your tax documents and worksheets.
