We measured CSF levels of beta-endorphin, an opioid hormone, in 19 patients with infantile autism and in 3 patients with Rett syndrome, and compared them with control values. In infantile autism, CSF levels of beta-endorphin did not differ significantly from those of age-matched controls. There was no significant correlation between CSF levels and clinical symptoms, including self-injurious behavior, pain insensitivity, and stereotyped movement. However, CSF levels of beta-endorphin were significantly higher in the patients with Rett syndrome than in the control (p < .05). Data suggest that neurons containing beta-endorphin may not be involved in patients with infantile autism. Thus, there is no relationship between dysfunction of brain opioid and autism.