BACKGROUND: To investigate the sexual dimorphism in human corpus callosum (CC), we measured the mid-sagittal area of corpus callosum and its seven sub-regions. METHODS: Eighteen men (23.0+/-9.1 years) and 16 women (27.3+/-11.3 years) underwent 1.6 mm thickness SPGR (spoiled gradient recalled in the steady state) MRI. All subjects are right handed and had no neurological or psychiatric diseases. The education level was controlled. Exact mid-sagittal image was obtained by image reconstruction and geometric correction. According to the Witelson's criteria, CC was divided into seven subregions (a1~a7 from anterior to posterior) and then normalized by mid-sagittal cerebral area (b1~b7), cerebral volume (c1~c7) and whole CC (d1~d7). In each subregion, the pixel number was counted. The difference of CC areas between men and women were tested by t-test and ANCOVA. RESULTS: The volume of the cerebrum (p < 0.001) and the mid-sagittal cerebral area (p=0.002) were significantly larger in men by t-test. b2 (normalized genu by mid-sagittal cerebral area, p=0.040), c2 (normalized genu by cerebral volume, p=0.015), cT (normalized whole CC by cerebral volume, p=0.023) were significantly larger in women. cT excluding c2 was not significantly different (p=0.065). But, there was no significant difference in a1~a7, aT with ANCOVA (adjusted by age and cerebral volume). a2 (r=0.493), b2 (r=0.655), b5 (r=0.591), b7 (r=0.526), bT (r=0.611), c2 (r=0.548), c5(y=0.522) and cT (r=0.495) had significantly higher correlation with age (all p < 0.05) in men. However, they had no significant correlation with age in women. CONCLUSIONS: In this preliminary study, we hypothesize the genu of CC is larger in women than in men. However, with age, men's CC has evolving growth pattern whereas women's CC remains relatively static.