The sample size of the study included 102 autistic patients and 112 controls from the ages of 6 months to 20 years. The autistic group ranged from high functioning (high IQ) to severely retarded (low IQ) and the control group were typically developing, exhibiting no n e u r o l o g i c o r b e h a v i o u r a l abnormalities. Measurements of the brain structures were performed via MRI by two independent clinicians. The results of this study revealed a statistically significant correlation between increasing age and increasing development of the brainstem and its components (midbrain, pons and medullar oblongata) and increasing development of the cerebellar vermis and its components (vermian lobules IV, VI-VII and VIII-X) amongst both groups. When comparing the posterior fossa brain structures, the autistic group showed a statistically significant smaller size than the control group. In contrast, the pons, vermian lobules I-V and VI-VII were significantly larger in the autistic group than the control. There were no significant differences between the groups with regard to the sizes of the brainstem , midbrain , m e d u l l a oblongata, entire cerebellar vermis and vermian lobules VIII-X. Overall, this study demonstrated increased size of brain structures with increased growth among both the autistic and the control group. When comparing the size of these structures between the groups, there was a difference in size with the posterior fossa structures being smaller in the autistic group and the pons, vermian lobules I-V and VI-VII being larger in the autistic group.