Recent reports describe wide variations in the size and shape of the human corpus callosum. To investigate genetic influences on this variability, magnetic resonance images from 5 pairs of monozygotic twins and 10 unrelated control subjects were analyzed. Measurements of size and shape revealed greater similarity in twin pairs than in randomly paired controls. The results are consistent with the view that the anatomy of the corpus callosum, while clearly influenced by nongenetic factors, is under considerable genetic control.