Several recent studies have described the benefits of fenfluramine for the symptomatic treatment of infantile autism. No large surveys of side effects of this drug have been reported in autistic children. To evaluate the untoward effects of fenfluramine in children with autism, 12 subjects were systematically studied. Medication was administered in a double-blind, placebo-controlled cross-over study. Parents were trained in monitoring untoward effects. These observations were compiled in detailed daily notes. In addition, four cases describing unusual effects found in a sample of 170 patients treated with fenfluramine are also reported. In the initial 2 weeks of active drug listlessness, food refusal, and stomach upset were frequently seen. A different pattern of untoward effects was seen in the final 14 weeks of treatment. Irritability, agitation, and crying along with continued food refusal were noted. The subjects lost 2.1% of body weight during active drug phase, but there was a rebound weight gain during the subsequent placebo phase. A thorough understanding of fenfluramine's side effects and adverse reactions is necessary so as to differentiate them from the multiple symptoms inherent in the syndrome of autism.