OBJECTIVE To determine whether there are non-motor regions of cerebellum in which sizeable infarcts have little or no impact on motor control. EXPERIMENTAL PROCEDURES We evaluated motor deficits in patients following cerebellar stroke using a modified version of the International Cooperative Ataxia Rating Scale (MICARS). Lesion location was determined using magnetic resonance imaging (MRI) and computerized axial tomography (CT). Patients were grouped by stroke location-Group I, stroke within the anterior lobe (lobules I-V); Group 2, anterior lobe and lobule VI; Group 3, posterior lobe (lobules VI-IX; including flocculonodular lobe, lobule X); Group 4, posterior lobe but excluding lobule VI (i.e. lobules VII-X); Group 5, stroke within anterior lobe plus posterior lobe. RESULTS Thirty-nine patients were examined 8.0+/-6.0 days following stroke. There were no Group 1 patients. As mean MICARS scores for Groups 2 through 5 differed significantly (one-way analysis of variance, F(3,35)=10.9, P=0.000 03), post hoc Tukey's least significant difference tests were used to compare individual groups. Group 2 MICARS scores (n=6; mean+/-SD, 20.2+/-6.9) differed from Group 3 (n=6; 7.2+/-3.8; P=0.01) and Group 4 (n=13; 2.5+/-2.0; P=0.000 02); Group 5 (n=14; 18.6+/-12.8) also differed from Group 3 (P=0.009) and Group 4 (P=0.000 02). There were no differences between Groups 2 and 5 (P=0.71), or between Group 3 and Group 4 (P=0.273). However, Group 3 differed from Group 4 when analyzed with a two-sample t-test unadjusted for multiple comparisons (P=0.03). Thus, the cerebellar motor syndrome resulted from stroke in the anterior lobe, but not from stroke in lobules VII-X (Groups 2 plus 5, n=20, MICARS 19.1+/-11.2, vs. Group 4; P=0.000 002). Strokes involving lobule VI produced minimal motor impairment. CONCLUSION These findings demonstrate that cerebellar stroke does not always result in motor impairment, and they provide clinical evidence for topographic organization of motor versus nonmotor functions in the human cerebellum.