Evidence of callosal dysfunction in patients with multiple sclerosis (MS) was examined using a test of bimanual coordination. MS patients were slower than non-patients on the Bimanual Coordination Test (BCT) on both unimanual trials (simple motor speed) and bimanual trials (intermanual coordination). Further, when compared to normals, MS patients exhibited a substantially greater difference between bimanual and unimanual response time, suggesting a deficit in interhemispheric motor interactions. A subgroup of MS subjects who showed markedly inefficient callosal transmission had previously been identified on the basis of abnormal evoked potentials (low amplitude cross-callosal evoked potentials). In comparisons of MS subgroups, the deficit in bimanual motor coordination was found only in MS patients with EP evidence of inefficient callosal transmission. These data support the conclusion that deficits in bimanual motor coordination occur in MS and that these deficits are related to callosal dysfunction.