We reviewed the phenomenology of obsessive-compulsive disorder (OCD) in 70 consecutive children and adolescents studied prospectively at the National Institute of Mental Health, Bethesda, Md, between 1977 and 1987. There is striking similarity between the clinical presentation of OCD in children and in adult patients. Washing, grooming, and checking rituals and/or preoccupation with disease, danger, and doubt account for the great majority of cases. Twenty-five percent of subjects had a first-degree relative with OCD. The fixed content and style of symptoms within and across subjects, and the identical presentation across a wide age range, suggest an ethological model for OCD.