Seventy-five autistic children, 52 males and 23 females, were admitted to the Iowa Autism Programme over a 3-year period. An overall male-female ratio of 2.26:1 was found, but in the patients with IQs less than 50, the ratio was 1.31:1. When IQ and receptive language functioning were considered together, significantly more females than males showed a more deviant form of autism. Autistic females were more seriously affected than autistic males; and more autistic females than males had IQs of less than 50 and evidence of cerebral dysfunction. However, when autistic males and females were closely matched in chronological age and receptive language functioning, the males and females were equally impaired in cognitive and perceptual-motor abilities. The findings suggest that the overall greater degree of morbidity in autistic females was accounted for by a significantly greater proportion of autistic males affected with a mild form of autism, whereas a significantly greater proportion of autistic females were affected with a more deviant form of autism. The findings were linked to the hypothesis of differential genetic loading in males and females.