As if filing your taxes weren't bad enough, millions of Americans will be crunching their numbers twice this year as the Alternative Minimum Tax intrudes on a growing percentage of middle-income earners.

The parallel tax system, first enacted in 1969, was created to ensure wealthier individuals did not abuse tax shelters and paid their fair share. But these days many say the AMT, which eliminates certain tax breaks, has strayed from its original mission.

Last year, for example, thousands of individuals with income below $50,000 paid the AMT. By 2010, as many as 35.5 million taxpayers may be subject to AMT, according to the National Taxpayer Advocate Report to Congress.

"Alternative minimum tax is a misnomer," said Claudia Hill, a spokeswoman for the National Association of Enrolled Agents. "It isn't an alternative, because you don't have a choice. And it isn't the minimum, because you end up paying higher taxes than you would have otherwise."

If you're below the AMT threshold, of course, you don't have to worry. But it's getting harder to avoid.

That's because exemption levels have scarcely changed in the 30 years since the tax was created. Greater numbers of people pay the AMT as inflation drives salaries up each year. Depending on your income, you pay AMT at either 26 or 28 percent of AGI.

"More and more people are subject to the AMT," said enrolled agent Tony Bardi, of Gresham, Ore. "But their circumstances and living standard haven't changed. It's not supposed to be a 'go out and get everybody' tax, but that's what it's doing."

Currently, exemption amounts are $24,500 for taxpayers who are married filing separately, $49,000 for those married filing jointly, and $35,750 for singles and heads of households. The Tax Relief Act of 2001 increased the exemption amount only temporarily (until 2004), and by modest amounts -- $4,000 on joint returns and $2,000 on all others.

If your income falls above those thresholds, that's the first sign you may be subject to the AMT, said Cindy Hockenberry, an enrolled agent with that National Association of Tax Practitioners. It's a good idea, in that case, to compute your taxes twice -- first under the AMT system and again using the regular tax system. You pay whichever is higher.

"Most people say, oh, well, 'I don't have any of the exemptions the IRS is talking about,' but they forget about personal exemptions," she said, noting that most tax software programs will automatically flag you if AMT is owed. "More often than not, you end up owing the AMT."

All too often, the AMT is so complicated that most taxpayers aren't able to calculate it without professional help. Between keeping records, studying the law, preparing and mailing the form, a taxpayer may need almost 12 hours just to file using the AMT, according to the National Taxpayer Advocate.

It should be noted, too, that some people are more likely than others to pay the AMT, said enrolled agent Frank Degen, of Setauket, NY, especially if they take certain deductions.

It's not supposed to be a 'go out and get everybody' tax, but that's what it's doing. Tony Bardi, EA

If you claim any of the following deductions, and your AGI is above the exemption level, you should double check to make sure you don't owe the alternative minimum tax:

You live in a highly taxed state, you had a very large miscellaneous deduction, you have a large family, or you refinanced and used the money for something other than to build or improve the house, and your income is above the exemption.

State taxes, deductible on Schedule A, are not deductible under the alternative minimum tax system. If you live in a highly taxed state and take a sizeable deduction, you may be responsible for the AMT.

The AMT also affects taxpayers with large families. "I've got a guy with 8 or 9 kids," said Bardi. "He had a $29,000 deduction for dependents. But add that back in, and he may pay AMT."

In addition, people who took large miscellaneous expense deductions and people who exercised incentive stock options may be subject to AMT, too.