The serotonin system has been implicated as a factor in some cases of autism since the finding in 1961 of elevated serotonin (5-hydroxytryptamine) levels in the blood of patients with autism. This has been clarified as elevation in the platelet content of serotonin. Subjects with elevated whole blood serotonin levels have been shown to have elevated platelet serotonin transport into platelets and decreased serotonin 5-HT2 receptor binding. Most individuals with autism who are treated with potent serotonin transporter inhibitors have a reduction in ritualistic behavior and aggression. Reduction of central nervous system serotonin, induced by acute tryptophan depletion, causes a worsening of stereotyped behavior. Recent developments in the molecular biology of serotonin receptors are reviewed.