The efficacy of the serotonin (5-HT) uptake inhibitor clomipramine in the treatment of self-injurious behavior (SIB) was tested in individuals with severe and profound mental retardation. Six of the 8 subjects who completed a double-blind, placebo-controlled crossover trial exhibited a clinically significant improvement (50% or greater reduction from placebo) in the frequency of SIB. Clomipramine treatment was also associated with improvement in SIB intensity, frequency of stereotypy and compulsions, teacher ratings of stereotypy and social withdrawal, and frequency of staff intervention required for problem behaviors. Adverse effects (seizure and tachycardia/agitation) occurred in 2 of the 8 subjects. These results represent the first controlled trial of a 5-HT uptake inhibitor in the treatment of SIB in mental retardation.