The number of American children and adolescents treated for bipolar disorder increased 40-fold from 1994 to 2003, researchers report Tuesday 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 is surprising to 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 children with the disorder obtain the treatment they need.

Other experts say bipolar disorder is overdiagnosed. The term, the critics say, has become a diagnosis du jour, 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.

The spread of the diagnosis is a boon to drugmakers, 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," 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.