We measured simple visual and auditory reaction time (RT) and movement time (MT) in 32 patients with olivopontocerebellar atrophy (OPCA) in comparison to 32 control subjects. In addition, we followed 2 approaches to radiologic assessment by computed tomographic scans: subjective (by inspection of films) and objective (by measurement of 4 radiologic ratios at the level of the posterior fossa and 1 ratio at the supratentorial level). All OPCA patients had various degrees of cerebellar atrophy and lengthened RT and MT in comparison to their controls. There were no significant differences in RT and MT performances in patients with mild-moderate versus those with severe cerebellar atrophy as assessed by inspection of their films. OPCA patients with severe versus mild-moderate atrophy evaluated by 3 measures, i.e., brainstem, brachium pontis and fourth ventricle ratios, presented few significantly lengthened RT and MT performances. In contrast, patients with severe atrophy revealed by the midbrain ratio had significantly lengthened RT and MT performances compared to those with mild-moderate atrophy assessed by this ratio on 7 of 8 measures; the 8th measure showed a borderline significant difference. This could be explained by the fact that atrophy at the midbrain level is the only one which involves dopaminergic, noradrenergic and glutamatergic structures and pathways.