A female child and her father with cerebellar ataxia and retinitis pigmentosa are described. The father's clinical onset was in middle age, the course of his disease mild and his pneumoencephalogram showed cerebellar atrophy. On the other hand, his daughter's clinical onset was in late infancy, her course was rapidly progressive with manifestations of brainstem dysfunction. She had abnormal brainstem auditory evoked potentials and the computerized tomography scan showed atrophy of the posterior fossa. Recently a paternal aunt developed cerebellar ataxia at the age of fifty. The unusual early age of onset of dominantly inherited cerebellar ataxia and the extreme variation in expression of clinical manifestation are discussed.