Potential participants will first attend a screening visit, which will include a psychiatric diagnostic interview, a practice session for swallowing pill capsules, a physical exam, an electrocardiogram (ECG), a blood test, and an assessment of pubertal stage. Females of childbearing age will also undergo a urine pregnancy test. In an initial double-blind study phase, eligible participants will be randomly assigned to receive either atomoxetine or placebo for 8 weeks. A baseline visit will include several rating scales, observations, and an interview to assess adaptive functioning. These measures and procedures will be used to keep track of symptoms, side effects, and behavior that could change during the study. Children who are assigned to placebo and do not notice an improvement in their ADHD symptoms will be given the opportunity to receive atomoxetine at the end of 8 weeks. Study visits will occur once a week for 4 weeks, and then every other week for the remainder of the 8 weeks. During these visits, many of the baseline questionnaires and interviews will be repeated. At the Week 8 visit, the physical exam, ECG, blood tests, and some baseline questionnaires will also be repeated. All children who respond well to atomoxetine may continue taking the drug for an additional 10 months. During this time, participants will report to the clinic once a month for the first 4 months, then once at the end of 7 months, and finally once at the end of 10 months. The same measures and procedures that were done during the 8-week phase will be done during the 10-month phase of this study.