Animal experiments suggest that neurochemical and anatomic asymmetries exist within the basal ganglia, particularly the globus pallidus, and that these asymmetries correlate with both preferred direction of rotation and limb preference in lever pressing. Human neurotransmitter studies have also revealed a significant asymmetry within the globus pallidus, the left containing greater amounts of dopamine and choline acetyltransferase than the right. Recent anatomic studies of human subcortical regions, limited to the striatum, have failed to show a size asymmetry. We examined 18 normal brains in subjects ranging in age from 26 weeks gestation to 86 years for volumetric asymmetry of the globus pallidus. We found a significant asymmetry, the left side measuring larger in 16 of 18 brains. Considering the role of the basal ganglia in motor control, this data may reflect a structural basis of either axial or limb motor dominance in humans. © 1988 American Academy of Neurology.