Cerebellar hemisphere size was calculated in 10 autistic and 8 normal control subjects by summing the cross-sectional areas of cerebellar hemisphere tissue measured on paramidline sagittal magnetic resonance images. The areas of two cerebellar vermal regions (lobules I through V and lobules VI through VII) were also measured using the midsagittal image. Our cumulative slice area measure of cerebellar hemisphere size was significantly smaller in the autistic subjects than in the control group. The cumulative slice area correlated positively with the area of vermal lobules VI through VII only in the autistic subjects. Our results indicated that the decreased size of the cerebellar hemispheres and vermal lobules VI through VII was associated with autism.