Studies of magnetic resonance images have revealed morphological disorders of the brainstem and cerebellum in autistic children and adults. When we studied development of the brainstem and cerebellum in autistic patients, we found that although the brainstem and cerebellum significantly increased in size with age in both autistic patients and controls, these structures were significantly smaller in autistic patients than in controls. The speed of development of the pons, the cerebellar vermis I-V and the cerebellar vermis VI-VII was significantly more rapid in autistic patients than in the controls. However, the speed of development of the other brain structures in the posterior fossa did not differ between autistic patients and controls. The regression intercepts of the brainstem and cerebellum as well as those of their components were significantly smaller in autistic patients than in controls. Results suggest that brainstem and vermian abnormalities in autism were due to an early insult and hypoplasia rather than to a progressive degenerative process.