Since recent reports suggest structural brain abnormalities in autistic patients, we analysed magnetic resonance imaging (MRI) scans of autistic children. Planimetric measurements were done on midsagittal MRI scans, produced with a 0.5 T superconducting magnet. Scans of 13 'high-level' autistic subjects were compared with 35 control MRI scans, read as anatomically normal by a neuroradiologist. Corpus callosal, fourth ventricular, cerebellar, cerebral, and cranial areas were measured. The fourth ventricle was found to be significantly larger in the autistic group. No other areas in the midsagittal scans differed statistically between groups. Results suggest that structures defining the fourth ventricle are anatomically altered in autistic patients.