The size of the midsagittal area of the human corpus callosum obtained from postmortem measurement varied with tested hand preference. The corpus callosum, the main fiber tract connecting the two cerebral hemispheres, was larger by about 0.75 square centimeter, or 11 percent, in left-handed and ambidextrous people than in those with consistent right-hand preference. The difference was present in both the anterior and posterior halves, but not in the region of the splenium itself. This callosal morphology, which varied with hand preference, may also be related to individual differences in the pattern of hemispheric functional specialization. The greater bihemispheric representation of cognitive functions in left- and mixed-handers may be associated with greater anatomical connection between the hemispheres. The naturally occurring regressive events in neurogenesis, such as neuronal cell death and axonal elimination, may be factors in the individual differences in brain morphology and in functional lateralization. Specifically, right-handers may be those with more extensive early elimination of neural components.