We describe a 38-year-old woman with absence of right cerebellar hemisphere incidentally discovered by MR imaging. No cerebellar abnormality was detected on neurological examination. Tissue probably representing dysgenetic cerebellar tissue with no corticomedullary differentiation was present, connected to the right superior cerebellar peduncle. Ipsilateral enlargement of the pons and cerebral peduncle were additional findings. Although the terms "aplasia" or "agenesis" have been used to describe this entity, intrauterine destruction is the presumed pathogenetic mechanism in our case, and therefore these terms have been avoided. Asymmetry of pons and mesencephalon may be related to compensatory reorganisation or to the impairment of sequential development of nuclei and neural tracts.