Several recent reports suggest cerebellar abnormalities in patients with autism. To further investigate the posterior fossa in vivo, we analyzed axial (transverse) and coronal magnetic resonance imaging (MRI) scans of autistic patients. The MRI scans were measured at life-size by planimetry. Axial MRI scans of "high level" autistic patients were compared with control scans; there were no differences in the fourth ventricles, vermes, cerebella, and cerebellar-pontine "complexes" between the groups. Coronal MRI scans were also studied. In the coronal scans, the cerebella of autistic patients were proportionally smaller and the fourth ventricles proportionally larger. This suggests that there are morphologic changes in cerebella of autistic children; such alteration may best be viewed in the MRI coronal plane.