Do you have trouble paying attention in school? Would you rather do 137 things at once than focus on one task a time? Does your body scream to jump up and run around every time you're asked to sit still? If so, the problem might be reflected in your brain.

You probably know at least a few kids who have trouble with concentration, self-control, and organization. (You might even be one of these kids yourself.) In fact, 3 to 6 percent of schoolchildren in the United States have attention deficit hyperactivity disorder (ADHD). Scientists want to know what goes on in the brains of people with ADHD so that they might find better ways to treat it.

In one recent study, researchers from the University of California, Los Angeles School of Medicine scanned the brains of 73 young people, ages 8 to18. Twenty-seven of them had ADHD. Forty-six did not.

Children and teenagers with ADHD had less tissue in certain parts of their brain, called the prefrontal and temporal lobes, the study found. These kids also had extra tissue called gray matter in the cortex, or outer layer, in the back of the brain.

The results so far are somewhat puzzling. No one yet has a clear idea of what size differences in certain brain features might have to do with attention disorders. Scientists need more clues to try to figure out why some kids can sit still in the library while others have to be out on the playground.—E. Sohn