In 37 autistic children and 67 normal control subjects, determinations of plasma free and total tryptophan and blood serotonin levels were made simultaneously in order to establish a relationship between these parameters and the clinical rating scales: Children’s Psychiatric Rating Scale (CPRS-1), Werry-Weiss-Peters Activity Scale (WWPAS), and Developmental Quotient (DQ). The plasma free tryptophan level was significantly higher in autistic children than in normal control subjects. There tended to be a significant positive correlation between the plasma free tryptophan level and CPRS-1 or WWPAS score and a negative correlation between the plasma free tryptophan level and DQ. The blood serotonin level was significantly higher in autistic children than in normal control subjects. No correlation was established, however, between the blood serotonin level and CPRS-1, WWPAS score or DQ, and hence the clinical symptoms. Nor was there a correlation between blood serotonin and free tryptophan levels in these children. These results suggest that autistic children have some defect in tryptophan-serotonin metabolism in the brain, which is responsible for the clinical manifestations and behavioral abnormalities of infantile autism.