The efficacy of the serotonin uptake inhibitor clomipramine in the treatment of stereotyped and related repetitive behavior disorders was tested in individuals with severe and profound mental retardation. A double-blind, placebo-controlled crossover trial of clomipramine was associated with significant reductions in the frequency and intensity of stereotyped behavior and teacher ratings of stereotypy, hyperactivity, and irritability as well as increased adaptive engagement and decreased staff intervention for nontargeted behavior problems. Adverse effects occurred in 3 of the 10 subjects. Of the 7 subjects who tolerated the drug, 6 exhibited a clinically significant improvement in one or more repetitive behaviors. Our results provide support for the hypothesis that clomipramine is effective in treating stereotyped and related behaviors associated with mental retardation.