Autosomal dominant cerebellar ataxia type I was diagnosed in three unrelated families from Martinique (French West Indies), and linkage to the locus for spinocerebellar ataxia 2 (SCA2) was established. Neuropathological findings in two patients were those of olivopontocerebellar atrophy without oligodendroglial cytoplasmic inclusions. Cerebellar ataxia was associated with hyporeflexia in 68% of 31 examined patients, with slowed and/or limited eye movements in 65% and with dementia in 29%. No patients had optic atrophy, pigmentary retinal degeneration, spasticity or parkinsonism. Mean age at onset was 33 +/- 16 years, and onset before the age of 20 years was correlated with a more rapid and severe course of the disease. Movement disorders, oculomotor disturbances, sphincter disturbances and cognitive impairment were significantly more frequent in early than in late onset patients. This explains why the phenotype was strikingly different in one family, in which mean age at onset was much earlier. Comparison with previously described SCA2 families indicated similarities, such as reduced saccade velocity, supranuclear ophthalmoplegia and decreased reflexes, although phenotypic heterogeneity remains the outstanding feature of this disorder.