Origin and ontogenesis of human brain laterally are unknown. Using in vivo magnetic resonance morphometry we measured cerebral hemispheric asymmetry of the plenum temporals, a structural substrate of functional laterality, in pairs of monozygotic twins concordant or discordant for handedness. In both groups, intraclass (i.e., within twin pair) correlations were low. The right-handers showed leftward asymmetry whereas the left-handers lacked asymmetry. The discordance for lateralized brain anatomy can be accounted for by ontogenetic models assuming twinning of an asymmetrical germ or differential action of nongenetic factors within twin pairs in utaro. The findings confirm a coupling of lateralized structure and function of the human brain. At least in monozygotic twins, early epigenetic factors must play a role in anatomofunctional laterality development.