Screening assessment: The child's parent and teacher/daycare provider, if applicable, will need to complete some forms that describe the child's problems with overactivity, impulsivity and inattention. If these forms indicate that the child may have ADHD the investigator will schedule outpatient-screening assessments. The research staff will evaluate the child to see if s/he has a Developmental Disorder (DD) or PDD and ADHD. Each child will receive a developmental assessment, and each parent will be interviewed about his or her child's behavior. The child's teacher will be given several rating forms to complete. The parents will be asked to complete some questions about the child's development. Both parents (if available) will be interviewed about their family histories. Some of the screening assessments will be videotaped/audiotaped. Each child will have a physical examination, an electrocardiogram (EKG), a urine test and a blood test. The doctor will ask the parents about his or her child's medical history.