This research focuses on the treatment of Attention Deficit Hyperactivity Disorder (ADHD) in very young children. The medication being used is methylphenidate (Ritalin); it is being studied to determine its safety and how well it works to treat ADHD in preschool-age children (3-5.5 year olds).

Young children age 3-5.5 years will be studied. The study includes treatment with different doses of methylphenidate and placebo; all children will receive active medication during the study. Prior to treatment, each child will receive a thorough psychiatric evaluation and each guardian will complete a 10-week workshop (2 hours/week). Throughout the study, parents and teachers will complete forms that report on the child's behavior and possible side effects. Participants will be monitored by regular visits with a study physician. Most participants will be involved in the study for up to 16 months to ensure optimal evaluation, dosing, and monitoring

Boys and girls who are:

• Between the ages of 3-5.5 years
• Qualified as having ADHD by our diagnostic evaluations and clinical staff
• Otherwise generally healthy
• Willing, and have parents that are willing, to attend all visits required by the study
• Enrolled in some type of day-program: day care, preschool, nursery school, kindergarten, for at least 2 half days/week
• In classrooms with teachers that are willing to participate by completing rating scale