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.