Blood platelet serotonin content was measured in 30 children with early infantile autism, as defined by Kanner, 30 age-matched normal subjects, and 45 children with various neurological and psychiatric disorders. Serotonin content in the autistic group was 980 +/- 357 ng/mg platelet protein (mean +/- standard deviation), a value significantly higher than that for normal children, 807 +/- 202 ng/mg (p less than .025). Autistic children under school age had higher platelet serotonin concentrations than other older autistic individuals. There was little correlation between age and serotonin levels in the normal children. Elevated serotonin was also seen in some of the non-autistic pathological group, who were disturbed and hyperactive. Elevated serotonin levels are not necessarily a specific biochemical finding for autistic children, but seem to be due to their behavioral distinction.