BACKGROUND Autistic disorder is characterized by a fundamental disturbance in social interaction, impairments in communication, and a markedly restricted repertoire of activities and interests. Abnormalities in the serotonin neurotransmitter system have been identified in some persons with autism. No consistently effective and safe drugs have been developed for treating the symptoms of autism. METHODS Thirty adults with autistic disorder completed a 12-week double-blind, placebo-controlled trial of the potent and selective serotonin uptake inhibitor fluvoxamine maleate. Behavioral ratings were obtained at baseline and after 4, 8, and 12 weeks of treatment. RESULTS Eight (53%) of 15 patients in the fluvoxamine-treated group were categorized as responders compared with none of 15 in the placebo group (P = .001). Fluvoxamine was superior to placebo in reducing repetitive thoughts and behavior (P < .001), maladaptive behavior (P < .001), and aggression (P < .03), and in improving some aspects of social relatedness (P < .04), especially language usage (P < .008). Treatment response was not correlated with age level of autistic behavior, or full-scale IQ. Other than mild sedation and nausea in a few patients, fluvoxamine was well tolerated. No dyskinesias, adverse cardiovascular events, or seizures occurred. CONCLUSIONS Fluvoxamine is more effective than placebo in the short-term treatment of the symptoms of autistic disorder in adults. Controlled studies of fluvoxamine and other potent and selective serotonin uptake inhibitors seem warranted in children and adolescents with autism.