Using high-resolution in vivo magnetic resonance morphometry we measured the midsagittal area of the corpus callosum and total forebrain volume in 120 healthy young adults (mean age (+/- s.d.) 25.7 +/- 4.7 years). The forebrain volume-adjusted size of the corpus callosum was larger in women than in men (32 mm2 mean difference; p = 0.011). Handedness had no effect in this measurement. The morphometric data confirm a gender difference in cerebral structural organization.