Based on the investigations which led to the dopaminergic theory of the psychoses, homovanillic acid (principal derivative from dopamine) levels in the urines of 37 autistic children and 11 normal children acting as controls were studied. The favorable action of vitamin B6 on autism was confirmed in 15 children. Vitamin B6 reduced homovanillic acid levels in 33 autistic children and increased them in all the control group children.