Signs of atrophy on cranial CT were investigated in 35 patients diagnosed as suffering from autosomal dominant (n = 21) or idiopathic (n = 14) cerebellar ataxia. Thirteen patients with a pure cerebellar syndrome were examined after at least 4 years of disease (mean duration 10.5 years) and were classified as cerebellar atrophy (CA). Twenty-two patients with additional non-cerebellar signs were classified as olivo-ponto-cerebellar atrophy (OPCA). Four (30%) of the patients with CA had atrophy of the brain stem in addition. Of the 22 patients with OPCA, 9 (40%) had atrophy of the cerebellum only. In patients with CA or OPCA correlation of clinical signs with severity of atrophy on CT was poor. Atrophy on CT often fails to differentiate autosomal dominant or idiopathic cerebellar ataxias in CA or OPCA: patients with CA can also have atrophy of the brain stem and patients with OPCA do not necessarily show brain stem atrophy.