Forty-six patients suffering from autosomal dominant cerebellar ataxia type I (ADCA I) underwent to a genotype-phenotype correlation analysis by molecular genetic assignment to the spinocerebellar ataxia type 1, 2, or 3 (SCA1, SCA2, SCA3) genetic locus and electro-oculography. Oculomotor deficits that are attributed to dysfunction of cerebellar structures occurred in all three mutations without major differences between the groups. Gaze-evoked nystagmus, however, was not found to be associated with SCA2. Square wave jerks were exclusively observed in SCA3. The gain in vestibulo-ocular reflex was significantly impaired in SCA3 and SCA1. In SCA3 the severity of vestibular impairment increased with CAG repeat length. Severe saccade slowing was a highly characteristic feature of SCA2. In SCA3 saccade velocity was normal to mildly reduced while SCA1 fell into an intermediate range. The present data show that each mutation is associated with a distinct syndrome of oculomotor deficits. Reduced saccade velocity and the absence of both square-wave jerks and gaze-evoked nystagmus allow one SCA2 to be distinguished from SCA3 patients in almost all cases. The eye movement disorder of SCA1 patients, however, overlaps with both SCA2 and SCA3.