A case report is reported of bilateral cerebellar hemispheric agenesis which was associated with secondary degeneration of cerebellofugal and cerebellopetal tracts. Somatotopic correlations between the cerebellar and the medullary olive lesions were obvious: preserved dorsal accessory olives pattern correlated with the spared vermis and normal medial accessory olives with the spared flocculi. Cerebellopetal degeneration was more difficult to analyse. The relation of cerebellar agenesis with basal ganglia abnormalities and microcephaly is discussed.