Left-to-right differences in cerebral structures of the posterior temporal and inferior parietal region were investigated in CT examinations from 500 cases, by anatomic analysis based on the medullary branches of cerebral white matter. On the left side, the planum temporale and middle temporal gyrus were positioned more posteriorly, the intraparietal sulcus was deeper and lower, and the folding of the inferior parietal lobule was more extensive. The lateralization of the planum temporale diminished gradually from 19 years of age downwards. An anatomic analysis of the medullary branches succeeded in revealing the cerebral asymmetry and is expected to be useful in magnetic resonance imaging.