We report on seven Japanese families with spinocerebellar ataxia type 6 (SCA6) carrying small CAG repeat expansions in the calcium channel alpha1A subunit gene. The number of the expanded CAG repeat, ranged from 22 to 25, showed no intergenerational instability and had a significant inverse correlation with the age of onset. The clinical features of these patients were late onset progressive pure cerebellar ataxia with dysarthria and nystagmus, and are consistent with autosomal dominant cerebellar ataxia type III (ADCA type III). Magnetic resonance imaging scan of the brain demonstrated cerebellar atrophy with no evidence of brainstem involvement. We propose that clinical phenotype of SCA6 is compatible with ADCA type III and SCA6 is one of the most common types of ADCA in Japan.