Cheating on your taxes is a bad idea. Paying someone else to cheat for you isn't any smarter.

It's also dumb to rely on professional preparers who claim that paying taxes is somehow voluntary -- or that your salary somehow doesn't represent taxable income. Or that you can safely inflate your charitable donations so you deduct the national average for your income level.

To be sure, most tax pros wouldn't dream of making such outlandish claims. In recent years, however, a small but growing number of tax preparers have landed themselves -- and often their clients -- in hot water for breaking the law on clients' returns. This trend has helped spark calls within Congress -- and also from a senior Internal Revenue Service official -- to expand regulation of paid preparers and crack down harder on offenders.

Pursuing "abusive" tax preparers is part of the IRS's stepped-up campaign against tax fraud and other forms of noncompliance. The agency continues to face intense pressure from Congress to cut the "tax gap," the difference between what IRS actually collects each year and what it thinks it should be collecting. Researchers have estimated the gap at about $290 billion, based on 2001 data.

The IRS's criminal investigative division recommended prosecutions of 196 paid preparers in fiscal 2007, which ended Sept. 30. That was up from 153 the year before and 140 in 2005. The number of preparers who were sentenced rose to 123 in fiscal 2007 from 109 in 2006 and 118 in 2005 -- and only 49 in 2003. The average number of months to serve rose to 19 in fiscal 2007 from 18 in each of the two previous years.

To be sure, arguing that you relied on a tax adviser may sometimes help you avoid the most severe problems. But it can still result in serious trouble -- and possibly even prison.

Just ask Wesley Snipes , the actor, who is awaiting sentencing after having been convicted last month on three misdemeanor counts for willful failure to file tax returns. He claimed he relied on advisers and was found not guilty on far more serious charges of federal tax fraud and conspiracy. Even so, he could go to jail for up to three years. Two other men who advised him were convicted of tax fraud and conspiracy.

"You need to vet your advisers. You need to check them out -- and, if someone promises you something that's out-of-this-world good, get a second opinion," says JJ MacNab , an insurance analyst in Bethesda, Md., who attended the Snipes trial and is writing a book on tax protesters.

The Snipes case has drawn close attention not only because of Mr. Snipes's celebrity but also because Congress and the IRS have been trying to crack down on people who make what government officials refer to as "frivolous arguments," including claiming not to be legally obligated to pay taxes, as Mr. Snipes once claimed.

So how worried should you be if your preparer does something illegal on a return that you sign and send in? In general, the answer is: "Very worried," replies Bryan C. Skarlatos, a tax lawyer at Kostelanetz & Fink LLP in New York.

Just how worried depends on the facts of your case -- such as whether you were an innocent victim or whether you were a knowing participant in the preparer's misconduct. For example, unless you are victimized by a preparer who stole your refund, "it's virtually certain you'll be liable for any back taxes, plus interest," Mr. Skarlatos says. "If you were involved in any wrongdoing, the IRS might impose stiff penalties -- and you may even face criminal charges, too."

At issue is "whether the taxpayer knew or should have known that their preparer cheated on the return," says Charles Rettig , a tax lawyer at Hochman, Salkin, Rettig, Toscher & Perez PC in Beverly Hills, Calif. Taxpayers need to remember that a tax return "is not an offer to negotiate with the government." Moreover, taxpayers "should not be intentionally ignorant." If a return includes questionable items, be sure to ask your preparer about them, he says.

Taxpayers may be able to avoid penalties if they can show they reasonably relied on the advice of a pro and had no way of knowing the advice was wrong, Mr. Skarlatos says. But that can often be tough to demonstrate, especially if the taxpayer is highly educated. (This issue can get extremely complex. If you have any doubts, consult an experienced lawyer.)

In a 2006 report, the U.S. Government Accountability Office cited major errors, including cheating, by paid preparers at outlets of several commercial tax-preparation chains when undercover investigators tested them. "In most states, anyone can be a paid preparer regardless of education, training, or licensure," Michael Brostek of the GAO said.

In a recent case in North Carolina, Lloyd Anthony Bastfield , who was a professional tax preparer, was sentenced to 70 months in prison and ordered to pay $6 million in restitution. He admitted filing returns claiming nearly $6 million in false claims for refunds for individuals over a four-year period -- and evading over $171,000 in personal income taxes he owed for four years. He admitted he had prepared and electronically filed more than 10,000 fraudulent returns for individuals claiming fictitious education income tax credits.

"A willful failure to pay taxes is not an oversight. It is a crime," says U.S. Attorney Gretchen C.F. Shappert. "Tax fraud is even more serious when it is committed by someone with a professional obligation to see that taxes are filed correctly." She also said individual clients who improperly got refunds as a result of Mr. Bastfield's conduct will have to pay back the government.

Mr. Bastfield's lawyer, John Nickerson in Charlotte, N.C., said his client isn't appealing the sentence. "What I want to say, honestly, is this is an unfortunate case," he says, "and it does show a need for there to be tighter controls on those who are allowed to prepare taxes."