The alternative minimum tax -- which is hitting a growing number of Americans--is a separate tax computation that, in effect, reduces the benefit of certain deductions and credits, thus creating a tax liability for an individual who would otherwise pay little or no tax. You may have to pay the alternative minimum tax if your taxable income for regular tax purposes, plus any of the adjustments and preference items that apply to you, is more than a specified exemption amount (for 2007, for married and surviving spouses, the amount is $45,000; lower amounts apply to other filing statuses). In other words, the AMT operates under many different rules than its counterpart.
To determine if you may be subject to the alternative minimum tax, see the Form 1040 instructions or refer to Form 6251, Alternative Minimum Tax - Individuals.