Scientists have discovered the first specific abnormality in the brains of people with hyperactivity, the most common psychiatric disorder of childhood, and believe it may an underlying cause of the condition.

But a Utah scientist said the basic cause of hyperactivity may lie elsewhere.The study found that parts of the brain essential for attention and controlling behavior do not work as hard as usual in hyperactive people. Experts described this as a potentially important insight into the sources of the mysterious ailment.

Dr. Paul H. Wender of the University of Utah said that finding any biological abnormality in a mental illness "is of great interest." But he said it may not turn out to be the basic cause of hyperactivity. Instead, the cortex may be sluggish because of faulty control signals from a more primitive part of the brain.

"If the more primitive centers don't tell the guys upstairs to wake up, they won't have attention," he said. "But the problem is not in the cortex. It's the guys downstairs."

People have long suspected that some physical irregularity is at the heart of the disorder, which affects between 2 percent and 4 percent of school-age children.

However, Dr. Alan J. Zametkin, who directed the research, said, "This is the first large study of hyperactivity that shows some sort of metabolic abnormality."

The research, conducted at the National Institute of Mental Health in Bethesda, Md., was published in Thursday's New England Journal of Medicine.

Brain scans of adults who had been afflicted with the disorder since childhood showed that their brain cells were 8 percent less active than those of normal people. However, the reduction was especially dramatic in the parts of the brain that control attention and inhibit behavior.

Hyperactivity, known technically as attention deficit-hyperactivity disorder, is a serious ailment that makes children inattentive and impulsive. They tend to blurt out answers in school, run into the street without looking and constantly fidget, squirm and jump about. Their hot tempers and frequent disobedience make them difficult for parents and teachers to handle.

About half the time, the disorder persists into adulthood, and victims are more likely to have run-ins with the law and abuse drugs and alcohol.

About three-quarters of victims improve when given mild stimulant drugs, such as Ritalin.

Zametkin said he hoped the latest findings will reduce the stigma attached to being treated for this disorder by showing that it involves a biological abnormality.

The researchers compared the metabolism - the rate at which energy is used - in the brains of the hyperactive adults and 50 normal people. They found that brain activity was significantly reduced among hyperactives in 30 of 60 specific regions of the brain.

In particular, they noticed reductions in the premotor cortex and the superior prefrontal cortex. These parts of the brain control attention. Damage to them can result in inattention and difficulty in controlling behavior.