Interest in anatomic evaluation of the corpus callosum is based upon the expectation that its function will be influenced by its structure. Using MR images, the size of the corpus callosum was determined by measuring its area in the midsagittal plane. A callosal index was calculated for each subject by dividing the callosal size by cerebral size, also measured from the MR image. This callosal index was designed to allow comparison of callosal size between different groups, despite differences in brain size that existed between the groups. The validity of this index was supported by the statistically significant, positive correlation between measured callosal area and cerebral size. Using MR images and the callosal index, adult males were shown to have a larger corpus callosum and larger cerebrum, compared to adult females. However, the callosal index was identical for male and female subjects. This demonstrated there relative size of the callosum was the same for males and females. Childhood development of the corpus callosum was documented through the first two decades of life by a progressive increase in the callosal index. In adults, cerebral atrophy caused a decline in callosal size, but no decline due to simple advancing age was found in normal adult subjects. This use of cerebral size to calculate a callosal index appeared to be useful in comparing callosal size between subjects and may be useful in future study of other disease states of the corpus callosum.