We measured CSF levels of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) by high-performance liquid chromatography in seven children with infantile spasms and in a group of age-and sex-matched controls. The mean concentration of the serotonin metabolite 5-HIAA was 40% (p < 0.01) lower in the infantile spasms group as compared with controls; HVA levels were similar in both groups. The data provide additional evidence that serotonin metabolism is abnormal inpatients with infantile spasms.