We compared horizontal eye movements (visually guided saccades, antisaccades, and smooth pursuit) in control subjects (n = 14) and patients with three forms of autosomal dominant cerebellar ataxias type I: spinocerebellar ataxias 1 and 2 (SCA1, n = 11; SCA2, n = 10) and SCA3/Machado-Joseph disease (MJD) (n = 16). In SCA1, saccade amplitude was significantly increased, resulting in hypermetria. The smooth pursuit gain was decreased. In SCA2, saccade velocity was markedly decreased. The percentage of errors in antisaccades was greatly increased and was significantly correlated with age at disease onset. In addition, a correlation between smooth pursuit gain and the number of trinucleotide repeats was found. In SCA3, gaze-evoked nystagmus was often present as was saccade hypometria and smooth pursuit gain was markedly decreased. Three major criteria, saccade amplitude, saccade velocity, and presence of gaze-evoked nystagmus, permitted the correct assignment of 90% of the SCA1, 90% of the SCA2, and 93% of the patients with SCA3 to their genetically confirmed patient group and, therefore, may help orient diagnoses of SCA1, SCA2, and SCA3 at early clinical stages of the diseases.