This occurs when an asset has one basis for income tax purposes and another for alternative minimum tax (AMT) purposes. In this situation, the regular tax gain or loss on a sale will not be the same as the AMT gain or loss. This situation can occur when you exercise and hold ISOs and trigger the AMT. You have to report an adjustment for AMT purposes, and your AMT basis in the stock is increased by the amount of the adjustment, while the basis for the ordinary income tax is just the exercise price.