Thirty-three outpatient epileptics with normal CT scans (group 1) and 31 patients with cerebellar and brain stem (CBS) atrophy (group 2) were randomly included in this study. There were no significant statistical differences between the groups with regard to age, education, and number of grand mal and other seizures. Statistical analyses showed that group 2 had a longer history of epilepsy with a consequently longer duration of phenytoin (PHT) consumption. Neuropsychological assessment revealed lower performance by this group on the following measures: full I.Q. scale, verbal I.Q. scale, performance I.Q. scale, information, arithmetic, block design, object assembly, digit symbol, Stroop test forms I and II, the B-M dexterity test, and the simple visual and auditory reaction time. No significant differences were observed between the two groups for the remaining 5 subtests from the WAIS scale, for the immediate recall and the delayed recall subtests belonging to Wechsler memory scale as well as for visual and auditory movement time. Analysis of the composite scores of neuropsychological performance showed that the cerebellum interferes with the following complex behavioral functions: (i) visuo-spatial organization for a concrete task, a function related to the cerebello-parietal loops' (ii) planning and programming of daily activities, a function related to the cerebello-frontal loops; and (iii) the speed of information processing, a mainly subcortical function.