The effects of fenfluramine were studied in a group of 44 children with the autistic syndrome and in 26 non autistic children with behavior abnormalities, mostly hyperkinetic children, as a control group. The mean daily dosage was 0.65 mg/kg/day. There were 75% positive results in the autistic children and 77% in the control group. The clinical improvement appears to be mainly related to the control of the hyperactive behavior in the autistic children. Platelet serotonin levels were studied in both groups, showing a clear cut decrease during fenfluramine therapy with no significant differences between the 2 groups and no correlation between the clinical effects and the magnitude of the decrease.