The idiopathic cerebellar ataxias (IDCA) comprise a wide spectrum of neurodegenerative diseases with heterogeneous neuropathology, characterized by the negativity of search for any known genetic mutation. On the basis of both their clinical presentation and their magnetic resonance imaging pattern, patients with IDCA can be subdivided into patients with a purely cerebellar syndrome and atrophy of the cerebellum (IDCA-C) and patients with additional noncerebellar symptoms and atrophy of both cerebellum and brainstem (IDCA-P). The aim of the present study was to evaluate the disaggregated contribution of brainstem and cerebellum in the control of eye movements, by means of an extensive battery of quantitative tests covering most oculomotor subfunctions related to lesions of the cerebellum and the brainstem. The smooth-pursuit movement analysis showed a decrease in gain and magnitude in both subgroups of IDCA with respect to normal controls, without any significant differences in the prevalence pattern between the two subgroups; the mean values of these parameters, however, were significantly lower in IDCA-P than in IDCA-C subjects in both gain (P < 0.01) and magnitude (P < 0.001). No statistically significant difference was observed between the two subgroups in the analysis of saccadic movements or in the other parameters investigated. The distinction between IDCA-P and IDCA-C subgroups has clinical implications, as a poorer prognosis is related to brainstem involvement, which may occur late in the course of the disease. Thus, the possibility to detect the brainstem involvement, also in association with cerebellar impairment, by a relatively simple eye-movement analysis, potentially useful mainly in follow-up investigations, needs to be evaluated further.