The authors reviewed all the population studies on autism published in the English language with particular reference to the rate of medical disorders. Seven studies met criteria for inclusion in the survey. The mean of possibly autism-related medical disorders in persons with autism across these studies was 24.4%. There was a trend for higher rates of medical disorders among subjects with severe mental retardation. The evidence in respect of atypical autism was equivocal, and the overall prevalence of medical disorders in this group was similar to that found in typical autism.