The clinical features are outlined and the neuropathological changes described in 3 cases (2 adults and 1 child) of Huntington's disease with severe atrophy of the cerebellum. Onset occurred at the ages of 56, 55 and 3 and death at 70, 62 and 6 years, respectively. All cases presented with cerebellar ataxia and this is also recorded in one relative of each adult case. The family history of Huntington's disease was not ascertained until the later stages of each patient's illness. At necropsy, the 3 cases showed the characteristic striatal and cerebral cortex atrophy and the shrunken cerebellum showed diffuse thinning of the molecular and granular layers with almost complete loss of Purkynĕ cells.