Through persistence and creativity, the health insurance industry has again found a novel way to avoid doing what it was established to do: cover sick policy holders. Insurers this time are counting on the widespread stigma against mental illness to allow it to offer lesser coverage to diseases of the brain compared with diseases of the rest of the body.

Congress thought it had taken care of the problem two years ago when it passed the Mental Health Parity Act, which explicitly was designed to eliminate the disparity of coverage between mental and physical ailments. But instead of simply lifting the dollar limits many insurance companies set for mental-health coverage, they also added a new kind of limit, capping the number of visits to outpatient treatment or the number of hospital days, either annually or lifetime, according to the New York Times.

The shift clearly violates the intention of the federal law. If these new limits were set not for mental health but for, say, cancer, the public would be outraged. The general uncertainty about the origin and effect of mental illness prevents similar outrage, instead too often evoking suspicion about mentally ill patients themselves. That could be changing — not because of a groundswell of empathy for sufferers of mental illness but through advances in science that raise doubts about whether these illnesses are psychological or physiological.

That is the case William Trask of Brewer will bring before the U.S. District Court this spring. Mr. Trask suffers from bipolar disorder — sometimes known as manic depression — that cost him the ability to work for his former employer, General Signal Corp. Once on long-term disability, he discovered that, for physical illnesses, he was covered until age 65, but the limit on coverage for mental illness was a mere 24 months. He sued and has been supported by the Maine Human Rights Commission.

Mr. Trask’s suit is based in part on the contention that, because bipolar disorder is caused by a chemical abnormality and can be successfully treated through medication, it, like many other illnesses considered purely mental, is in fact a physical limitation and, therefore, should be fully covered.

There is a danger in this approach, however. It risks denying the validity of illnesses that cannot (yet) be broken down into chemical components. Just as viruses were quite real before their existence was demonstrated, origins of mental illnesses may be as tangibly real but so far elusive. Nevertheless, the point the suit raises is important: Mental illness is every bit as debilitating as a broken leg and deserves to be covered as completely as any obvious physical illness.

Congress can write another law that closes the loophole created by insurance companies. Sen. Paul Wellstone of Minnesota already has proposed “full parity” legislation that will be considered this winter. But the threat of insurers simply finding another way around actually insuring the people who pay the policies deserves further investigation. It invites serious penalties for those who flout the spirit of the law and harm not only the person denied coverage but, by perpetuating a stigma, all mentally ill people.