Four persons who exhibited prominent autistic features throughout life died when 4, 14, 27, and 33 years old. All were mentally retarded. One had documented phenylketonuria, but the cause of mental retardation and autistic behavior was undefined in three. At the time of autopsy, brain weights were within 2 SDs of the norm for age. Complete neuropathologic examination, including analysis of cortical neurons impregnated with the rapid Golgi method, failed to provide clues as to cause or the pathoanatomic substrate of autistic behavior in these cases.