The clinical features of 36 patients with late onset cerebellar ataxia of unknown cause are described. Overall, the age of onset ranged from 30 to 74 years and there was a significant excess of males. The patients were divided into 3 groups on clinical grounds. The first was composed of 12 cases in whom truncal ataxia was more marked than limb ataxia and onset was relatively late (mean 54.75 years); these correspond to the Marie-Foix-Alajouanine type of cerebellar degeneration. The second group contained 6 individuals who had prominent tremor in the upper limbs, both resting and during action. The 18 individuals in the 3rd group were clinically similar to patients previously reported as sporadic examples of olivopontocerebellar atrophy. It was this latter category which contributed to excess of males. None of the patients had similarly affected relatives. Both the 3rd groups, and all 36 cases were compared wih 37 other patients with dominantly inherited late onset cerebellar ataxia in order to establish which clinical features might indicate the presence of new dominant mutations in the "sporadic" cases. Optic atrophy ophthalmoplegia and pigmentary retinal degeneration were more frequent in the familial cases.