Variation in the size of the human corpus callosum was examined in relation to variation in measured IQ. The midsagittal surface area of the corpus callosum, obtained by magnetic resonance imaging, was measured in 47 patients with epilepsy. Intellectual ability was positively related to a larger posterior callosal area. We suggest that the relationship between the posterior callosal region and measured intelligence is "non-functional" in itself, but rather, may reflect other anatomical-cognitive associations. That is, differences in splenial size may reflect differences in the number of cortical neurons and interconnections between areas of the brain that are important for processing the kind of information measured on intelligence tests. Our conclusions, however, must be tempered by a number of factors; in particular, the nature of our subjects and the relatively small sample size.