Whole blood and urinary levels of serotonin (5-hydroxytryptamine; 5-HT) and the derivative urinary 5-hydroxyindoleacetic acid (5-HIAA) were measured in normal and autistic subjects. An association was tested between autism and a marker coding for the 5-HT2A serotonergic receptor gene. Significant group (high urinary 5-HT and low whole blood 5-HT in autism) and age effects (urinary 5-HT decrease with age) were found. Moreover, whole blood 5-HT levels were correlated with clinical state. No differences in allele and genotype frequencies for the 5-HT2A receptor marker were found in this autistic population compared with age-matched healthy students.