Letter to the Editor in response to a report by Geller et al. The article presented preliminary results suggesting the possible usefulness of fenfluramine (a substituted phenylethylamine widely used as an appetite suppressant) in the treatment of infantile autism (July 15, 1982, issue). The rationale for the use of this agent rests on its ability to lower peripheral-blood levels of serotonin and on the observation that a substantial minority of autistic persons have elevated peripheral-blood serotonin levels. The report emphasized the preliminary nature of the results in a small sample of three young autistic boys with elevated serotonin levels