A sizable number of school-aged children, more often boys, may have attention-deficit hyperactivity disorder (ADHD). Its cause is unknown. Some investigators implicate food additives or lead poisoning. These theories are not supported by study results, however. Physicians are dependent on reports of parents and teachers to diagnose ADHD, since the children may not show signs at an office visit. The presence of criteria listed in the Diagnostic and Statistical Manual of Mental Disorders, third revised edition, and descriptions by parents and teachers of significant and pervasive impairment at home or school are diagnostic. Treatment is essential to minimize the academic failure, rejection, and loss of self-esteem that can result from the behaviors that typify ADHD. Use of stimulant medication is the most effective intervention and should be continued as long as it provides benefits without adverse effects. Concomitant behavioral therapy may be useful, and the entire family may benefit from psychotherapy.