Seventy-seven families with autosomal dominant cerebellar ataxia were analyzed for the CAG repeat expansions causing spinocerebellar ataxia (SCA) types 1, 2, 3, and 6. The SCA1 mutation accounted for 9%, SCA2 for 10%, SCA3 for 42%, and SCA6 for 22% of German ataxia families. Seven of 27 SCA6 patients had no family history of ataxia. Age at onset correlated inversely with repeat length in all subtypes. Yet the average effect of one CAG unit on onset age was different for each SCA subtype. We compared clinical, electrophysiological, and magnetic resonance imaging (MRI) findings to identify phenotypic characteristics of genetically defined SCA subtypes. Slow saccades, hyporeflexia, myoclonus, and action tremor proposed SCA2. SCA3 patients frequently developed diplopia, severe spasticity or pronounced peripheral neuropathy, and impaired temperature discrimination, apart from ataxia. SCA6 presented with a predominantly cerebellar syndrome and patients often had onset after 55 years of age. SCA1 was characterized by markedly prolonged peripheral and central motor conduction times in motor evoked potentials. MRI scans showed pontine and cerebellar atrophy in SCA1 and SCA2. In SCA3, enlargement of the fourth ventricle was the main sequel of atrophy. SCA6 presented with pure cerebellar atrophy on MRI. However, overlap between the four SCA subtypes was broad.