The cross-sectional areas of the corpus callosum (CC) and anterior commissure (AC) were determined by computer-assisted morphometry in normal human brains obtained at autopsy. In addition, the shape of each CC was examined qualitatively by three "blind" observers. A two-fold variation was observed in the cross-sectional area of the CC. Surprisingly, callosal cross-sectional area was not significantly related to brain weight. Moreover, contrary to recent reports, neither simple inspection nor morphometry revealed structural variation related to sex. A striking, seven-fold, variation was observed in the cross-sectional area of the AC. However, AC cross-sectional area was not related either to brain weight or CC cross-sectional area. A trend toward sexual dimorphism in AC cross-sectional area was observed, with males having the larger AC's. Since the interhemispheric commissures are composed, to a large extent, of fibers that link the various cortical areas of the two hemispheres, these observations suggest that variation in the cross-sectional area of the interhemispheric commissures is not simply related to brain weight or sex but, rather, reflects a similar degree of variation in some aspect of cortical structure.