The purpose of this review is to summarize existing data on the long-term safety and efficacy of stimulant treatment, and how long-term stimulant treatment of children with attention deficit hyperactivity disorder (ADHD) affects their outcome. Existing controlled studies of children with ADHD treated and untreated with stimulants, as well as long-term prospective follow-up studies, are reviewed. Children with ADHD treated with stimulants for as long as 2 years continue to benefit from the treatment, with improvements observed in ADHD symptoms, comorbid oppositional defiant disorder, and academic and social functioning, with no significant problems of tolerance or adverse effects. Long-term, prospective follow-up studies into adulthood show that stimulant treatment in childhood has slight benefits regarding social skills and self-esteem. Long-term adverse effects from stimulant treatment in childhood regarding adult height or future substance abuse have not been supported by existing studies.