For Frank McGrath, a 38-year-old mechanic in Silver Spring, the news that he has attention deficit hyperactivity disorder came as a great relief.

"Before I was diagnosed I always thought that I was stupid, that I was sentenced to a lifetime of menial jobs, but now I know that I could go back to college," said McGrath, one of the more than 1 million adults believed to suffer from the problem, which is characterized by a profound restlessness and an inability to concentrate.

McGrath, who is among 75 adults who participated in an important study of the disorder at the National Institute of Mental Health, exemplifies how the problem doesn't necessarily end in childhood. About 60 percent of the estimated 2 million children believed to suffer from the disorder are plagued by life-long distractability and impulsiveness -- traits that can have disastrous consequences.

In the past five years, McGrath said he was fired from two jobs because he couldn't concentrate long enough to complete a mechanic's checklist. He has forgotten to pay bills and taxes -- oversights, he said, that have left him with a poor credit rating. He said that his lack of concentration and impulsive behavior were factors in the break-up of his first marriage and have caused strains with his second wife.

McGrath's experience also illustrates the genetic component of hyperactivity: His 12-year-old son Blaine has the same problem. An estimated 30 percent of affected children have a parent or sibling who suffers from the disorder, and often it is the child's problems that lead to diagnosis in a parent, as was the case with the McGraths.

Last week, after studying McGrath and other adults with the condition, NIMH researchers offered the first substantive evidence that hyperactivity is not just a psychological problem, but appears to have a biological basis. Their report, published last week in the New England Journal of Medicine, found that in people with attention deficit hyperactivity disorder, key regions of the brain function abnormally. Researchers hope that this finding may lead to a diagnostic test for the condition.

According to Alan J. Zametkin, the study's principal author who is senior staff psychiatrist in NIMH's section on clinical brain imaging, adults with the disorder were slower to metabolize glucose, the brain's main energy source, than did adults who were not hyperactive.

The report was a relief to many families. "All these parents who have been blaming themselves because their children have attention deficit disorder can now know that it is not their fault," said Joan McGrath, Frank's wife and Blaine's stepmother.

It may also help to change the perception of children with the disorder. Two years ago, the seven year old son of Randi Thomas of Anne Arundel County, was diagnosed as hyperactive. Since then, Thomas said, she has spent much of her time educating the school system about the condition and dissuading her son's grandparents from the notion that "spanking him more often would improve his behavior."

Parents and teachers say that coping with hyperactive children is difficult. They throw frequent temper tantrums and experience wide mood swings and seem unceasingly restless. Most have trouble following instructions. A stimulant known as Ritalin, or methylphenidate hydrochloride, is commonly prescribed to treat the condition in children and adults, but its use is controversial.

"Before he was on medication, I would wake up Blaine in the morning, tell him to get up, get dressed, eat breakfast, put on his jacket and get his books for school -- all things that an average 12-year-old can do," Joan McGrath recalled. "But by the time he got to the kitchen, he had forgotten what he was there for."

Such distractability often is misinterpreted by parents and teachers "as being stubborn and oppositional," said Paul H. Wender, director of psychiatric research at the University of Utah Medical Center and author of a book on hyperactivity. "They get negative feedback from peers, at school and at home, which leads to low self-esteem."

Adults have similar problems. At work, McGrath, who repairs equipment, was regarded as unreliable. While driving from one customer to another, he often got distracted and decided to check out the latest model train magazines.

Even simple tasks, like fixing a light, became complicated. "When a light is broken, most people think about checking to see if the bulb is burned out first," he said. "I think about taking apart the light switch."

First described in 1922 in a group of children recovering from a viral brain infection, the disorder variously has been called minimal brain dysfunction, hyperactivity and attention deficit disorder. In 1987, the American Psychiatric Association renamed the condition as attention deficit hyperactivity disorder.

Diagnosis remains a problem. Doctors still have no good test for the condition and must rely on interviews with parents, teachers and the affected child.

Boys are three times more likely than girls to suffer from hyperactivity and are ten times more likely to require drug treatment, according to experts. The reason: "Girls with the condition don't tend to be aggressive," said Joseph Biederman, director of Pediatric Psychopharmacology at Massachusetts General Hospital in Boston.

Hyperactivity often overlaps with other problems. Learning disabilities, such as dyslexia, compound the problem of school failure and children whose symptoms continue into adolescence are at greater risk for drug and alcohol problems, according to Utah's Wender.

No one knows exactly what causes the condition; among the theories that have been considered are food additives, viral infections, too much dietary sugar, lead poisoning, head trauma and dysfunctional families.

Whatever the cause, the best treatments so far have been low doses of stimulants. Ritalin is prescribed most often and Wender said that about 75 percent of children and adults who are hyperactive "show a moderate to marked response to the drug," Wender said. Other drug treatments include the amphetamine dexedrine.

Despite its success, drug therapy is controversial. One concern is that it can markedly slow growth in children. Another is a link between the use of Ritalin and the subsequent development of Tourette's Syndrome -- a serious neurological tic disorder that doesn't always disappear when the drug is stopped. An estimated 60 percent of children with Tourette's are first diagnosed as having attention deficit hyperactivity syndrome and some doctors believe that Ritalin may help trigger Tourette's.

It is not clear why stimulants help control hyperactivity. Zametkin plans to study how drugs such as Ritalin work in the brain. Doctors theorized that the stimulants might increase energy use in some areas of the brain. Preliminary results from studies show that this doesn't happen. Instead, it seems that Ritalin helps even out the energy imbalance in portions of the brain.