For years, the public has used consumer-protection laws to fight unscrupulous business practices by used-car salesmen and mail-order vendors.

But soon consumers could have a new target: lawyers.

In Illinois, the daughter of a woman with Alzheimer's disease has gone to court against a lawyer who allegedly overbilled her mother by $40,000. The suit contends that by sending out inflated bills, the lawyer not only committed malpractice but also violated the state's tough consumer-protection law.

The case of 85-year-old Roberta Schmitz, which is headed to Illinois's highest court, raises a novel question: Should lawyers and other professionals be subject to the same laws as car dealers and home-improvement contractors?

In several places, including Washington state, clients can already sue lawyers under state business laws more easily than before. If the Schmitz suit succeeds in Illinois, the case, along with several others percolating nationally, could redefine how lawyers are regulated.

"The idea that lawyers should be above a law that applies to every other business person is patently absurd," contends James Turner, executive director of HALT, a Washington-based group that focuses on consumer-oriented legal issues and has filed court papers in support of the Schmitz suit.

Consumer-protection cases are unusually plaintiff-friendly. Unlike standard malpractice cases, they offer the possibility of large punitive damages. In contrast with fraud cases, they don't require clients to prove that their lawyers intended to deceive them - only that they were given misleading information or otherwise unin- tentionally misled. As a further incentive, the laws generally require defendants to pay consumers' legal fees if the consumers win in court.

In Illinois, the 35,000-member State Bar Association opposes the Schmitz suit, arguing that using the consumer-protection law, against lawyers would interfere with the state's lawyer disciplinary system, which regulates the practice of law in Illinois.

"This is a question of reasonable public policy-making," says Dennis Rendleman, the association's general counsel, adding that the bar group isn't taking a position on the merits of the Schmitz claim. "Who calls the balls and strikes?"

But proponents of broadening consumer-protection laws say the current system is stacked in favor of lawyers. In 1997, only 117 of the more than 4,000 attorneys investigated by Illinois disciplinary authorities were sanctioned or disbarred.

Traditionally, lawyers, doctors and clergymen haven't had to worry about suits alleging unfair or deceptive business practices because courts deemed them members of "learned professions," subject to different ground rules. The landscape started changing in the late 1970s, after the U.S. Supreme Court ruled that lawyer's could be sued for violating antitrust laws in a case challenging a minimum fee schedule set up by the Virginia bar.

In recent years, states have stripped away other protections. Two years ago, the Supreme Court of Connecticut held that doctors could be sued under that state's consumer-protection law for "entrepreneurial" activities, including competing for patients.

Mrs. Schmitz's ordeal began in 1992, when her husband died, leaving her two trusts valued at nearly $600,000. She hired Thomas Leiter, a lawyer in Peoria, III., who over about two years billed $65,000 in fees. The suit contends that about $40,000 of that sum was excessive or fraudulent.

David Sinn, an attorney for Mr. Leiter, denies that his client overbilled anyone and says Mr. Leiter got caught in the middle of an acrimonious family feud over control of the trust money.

"Tom had to do a tremendous amount if hand-holding," and research "all kinds if esoteric legal questions," he says, adding that Mr. Leiter also fought off an early attempt by certain family members to have Mrs. Schmitz declared incompetent.

Eventually, a court in Michigan, where Mrs. Schmitz now resides, determined that she was incapable of handling her affairs; a doctor had found her easily manipulated. A daughter of Mrs. Schmitz was appointed guardian, and she sued Mr. Leiter soon afterward.

Bradley McMillan, the daughter's lawyer, had never sued a fellow attorney before and knew about the consumer-protection law primarily from a defense angle, having previously represented insurance companies. "It just seemed very logical" that the law's ban on deceptive practices would apply to lawyers, he says.

In court papers, Mr. Leiter is accused of inadequately documenting his bills and including false information about the number of hours he worked. A separate suit against a bank that administered the trusts-and paid Mr. Leiter-was recently settled.

In 1996, a trial court dismissed the consumer-protection claim against the attorney. But last July, the ruling was overturned by an appeals court, which found no reason to exempt the "business aspects" of law from regulation under the consumer law. "The only skills required in billing," the court concluded, "are the ability to tell time." The state supreme court is expected to hear Mr. Leiter's appeal of that decision this spring.