This article outlines the value and limitations of the research in the area of long-term outcome of attention deficit and hyperactivity disorder (ADHD). It examines the natural progression of the disorder, and the factors affecting outcome. Early research findings since the 1960s have focused on ADHD primarily as a childhood disorder. More recently, a number of researchers have tracked Patients with ADHD longitudinally. These studies have been able to chart the natural progression of this disorder, ascertaining its continued presence in adolescence and adulthood, as well as to identify stable predictors of outcome. Results of long-term follow-up studies showed that in adolescence, most patients (70%-80%) continue to show symptoms of the disorder and continue to meet the diagnostic criteria for ADHD. In adulthood, many patients continue to be symptomatic (60%), but fewer meet the diagnostic criteria for ADHD. Research in this area is plagued by a number of methodological difficulties. In addition to the reclassification of the disorder over the years, differences in study designs have made it difficult to replicate key findings. Despite these difficulties, a number of consistent findings have been documented. The core symptoms of hyperactivity-impulsivity tend to decrease over time, although inattention may persist. Additional difficulties resulting from secondary problems often develop in later life. These difficulties include low self-esteem, poor academic performance, and poor interpersonal skills. Antisocial behavior and substance abuse in late adolescence and adulthood are important problems in some of these patients. The prognosis for these patients is influenced by the severity of symptoms, comorbidity, I.Q., family situation such as parental pathology, family adversity, socioeconomic status, and treatment. Treatment, particularly stimulant medication, can be helpful in the short term for these patients, but the long-term impact of treatment is unclear.