Autism, like mental retardation, is not a definitive diagnosis. This paper discusses the medical differential diagnosis to consider when patients present with autistic symptoms. Summarizing the medical literature, the known infectious, metabolic, chromosomal and structural lesions are reviewed. Autism also can be associated with sensory handicaps, such as blindness and deafiness. When the autistic syndrome is associated with a second syndrome, this produces a "double syndrome" in the patient. A thorough medical evaluation of all individuals with autism is suggested.