The number of American children and adolescents treated for bipolar disorder increased 40-fold from 1994 to 2003, researchers report today in the most comprehensive study of the controversial diagnosis.

Experts say the number has almost certainly risen further since 2003.

Many experts theorize that the jump reflects that doctors are more aggressively applying the diagnosis to children, and not that the incidence of the disorder has increased.

But the magnitude of the increase surprises many psychiatrists. They say it is likely to intensify the debate over the validity of the diagnosis, which has shaken child psychiatry.

Bipolar disorder is characterized by extreme mood swings. Until relatively recently, it was thought to emerge almost exclusively in adulthood. But in the 1990s, psychiatrists began looking more closely for symptoms in younger patients.

Some experts say greater awareness, reflected in the increasing diagnoses, is letting youngsters with the disorder obtain the treatment they need.

Other experts say bipolar disorder is overdiagnosed. The term, the critics say, has become a catchall applied to almost any explosive, aggressive child.

After children are classified, the experts add, they are treated with powerful psychiatric drugs that have few proven benefits in children and potentially serious side effects like rapid weight gain.

In the study, researchers from New York, Maryland and Madrid analyzed a National Center for Health Statistics survey of office visits that focused on doctors in private or group practices. The researchers calculated the number of visits in which doctors recorded diagnoses of bipolar disorder and found that they increased, from 20,000 in 1994 to 800,000 in 2003, about 1 percent of the population under age 20.

The spread of the diagnosis is a boon to drug makers, some psychiatrists point out, because treatments typically include medications that can be three to five times more expensive than those for other disorders like depression or anxiety.

“I think the increase shows that the field is maturing when it comes to recognizing pediatric bipolar disorder, but the tremendous controversy reflects the fact that we haven’t matured enough,” said Dr. John March, chief of child and adolescent psychiatry at the Duke University School of Medicine, who was not involved in the research.

“From a developmental point of view,” Dr. March said, “we simply don’t know how accurately we can diagnose bipolar disorder or whether those diagnosed at age 5 or 6 or 7 will grow up to be adults with the illness. The label may or may not reflect reality.”

Most children who qualify for the diagnosis do not proceed to develop the classic features of adult bipolar disorder like mania, researchers have found. They are far more likely to become depressed.

Dr. Mani Pavuluri, director of the pediatric mood disorders program at the University of Illinois, Chicago, said the label was often better than any of the other diagnoses often given to difficult children.

“These are kids that have rage, anger, bubbling emotions that are just intolerable for them,” Dr. Pavuluri said, “and it is good that this is finally being recognized as part of a single disorder.”

The senior author of the study, Dr. Mark Olfson of the New York State Psychiatric Institute at the Columbia University Medical Center, said, “I have been studying trends in mental health services for some time, and this finding really stands out as one of the most striking increases in this short a time.”

The increase makes bipolar disorder more common among children than clinical depression, the authors said. Psychiatrists made almost 90 percent of the diagnoses, and two-thirds of the young patients were boys, said the study, published in the September issue of The Archives of General Psychiatry.

About half the patients were identified as having other mental difficulties, mostly attention deficit disorder.

The children’s treatments almost always included medication. About half received antipsychotic drugs like Risperdal from Janssen or Seroquel from Astrazeneca, both developed to treat schizophrenia.

A third were prescribed so-called mood stabilizers, most often the epilepsy drug Depakote. Antidepressants and stimulants were also common.

Most children took a combination of two or more drugs, and 4 in 10 received psychotherapy.

The regimens were similar to those of a group of adults with bipolar diagnoses, the study found.

“You get the sense looking at the data that doctors are generalizing from the adult literature and applying the same principles to children,” Dr. Olfson said.

The increased children’s diagnoses reflect several factors, experts say. Symptoms appear earlier in life than previously thought, in teenagers and young children who later develop the full-scale disorder, recent studies suggest.

The label also gives doctors and desperate parents a quick way to try to manage children’s rages and outbursts in an era when long-term psychotherapy and hospital care are less accessible, they say.

In addition, drug makers and company-sponsored psychiatrists have been encouraging doctors to look for the disorder since several drugs were approved to treat it in adults.

Last month, the Food and Drug Administration approved one of the medications, Risperdal, to treat bipolar in children. Experts say they expect that move will increase the use of Risperdal and similar drugs for young people.

“We are just inundated with stuff from drug companies, publications, throwaways, that tell us six ways from Sunday that, Oh my God, we’re missing bipolar,” said Dr. Gabrielle Carlson, a professor of psychiatry and pediatrics at the Stony Brook University School of Medicine on Long Island. “And if you’re a parent with a difficult child, you go online, and there’s a Web site for bipolar, and you think: ‘Thank God, I’ve found a diagnosis. I’ve found a home.’ ”

Some parents whose children have received the diagnosis say that, with time, the label led to effective treatment.

“It’s been a godsend for us,” said Kelly Simons of Montrose, Colo., whose son Brit, 15, was prone to angry outbursts until given a combination of lithium, a mood stabilizer, and Risperdal, which was often given to children “off label,” several years ago. He now takes just lithium and is an honor roll student.

Other parents say their children have suffered side effects of drugs for bipolar disorder.

Ashley Ocampo, 40, of Tallahassee, Fla., whose 8-year-old son is being treated for bipolar, said that he had tried several antipsychotic drugs and mood stabilizers and that he had improved.

“He has gained weight,” Ms. Ocampo said, “to the point where we were struggling find clothes for him. He’s had tremors and still has some fine motor problems that he’s getting therapy for. But he’s a fabulous kid. And I think, I hope, that we’re close to finding the right combination of medications to help him.”