This report provides preliminary evidence for the efficacy of clomipramine in the treatment of young adults with autistic disorder. Four of five outpatients with autistic disorder showed significant improvement in social relatedness, obsessive compulsive symptoms, and aggressive and impulsive behavior with clomipramine treatment. These findings are consistent with previous evidence, suggesting that serotonin neurotransmission may be relevant to the treatment, and possibly the pathophysiology, of some symptoms of autistic disorder.