We administered secretin intravenously to 14 patients with autism (9 to 14 years, 10 males; 4 females), and evaluated its clinical effect. We also measured cerebrospinal fluid (CSF) levels of homovanillic acid (HVA) and 5-hydroxy-indole-3-acetic acid (5HIAA) before and after 4 weeks treatment, and compared them with the grade of improvement of the clinical symptoms assessed by the scores of Autism Diagnostic Interview-Revised (ADI-R). After injection of secretin, the ADI-R score increased in 8 patients, but declined in 3. Improvement was observed in functions such as sociability (interpersonal relationships), communication and speech improved, whereas in the others. symptoms such as hyperkinesias and stereotyped behavior became worse. The CSF levels of HVA was significantly increased in all of the patients showing an improvement in the ADI-R score. SHIAA levels also tended to increase, although this increase was not significant. These findings suggest that secretin promotes the metabolism of serotonin and dopamine in the central nervous system, which may contribute to improvement in clinical symptoms of autism.