MRI was performed for 18 autistic patients, 11 patients with uncomplicated mental retardation (MR) and 18 controls. In autism and MR, and altered left/right relationship of the frontal lobe volume was observed. The altered asymmetry was more severe in autism than in MR. The brain stem was smaller in MR than in the controls, and the right operculum was smaller in MR than in autism. These findings are consistent with the hypothesis that some autisms may involve a type of early developmental abnormality. Furthermore, our results suggest that autism may involve a type of structural brain impairment different from MR.