Total midsagittal area and seven subdivisions of the corpus callosum were measured on magnetic resonance images of 114 healthy boys and girls, aged 4 to 18. Striking variability of size was noted for all measures. Total midsagittal corpus callosum area increased in a robust and linear fashion from ages 4 to 18 (slope = 13.1 mm2/year, P = 0.0001 and slope = 11.1 mm2/year, P = 0.0001 for females and males, respectively). Posterior and mid regions demonstrated greater age-related changes than anterior regions with the rostrum and genu (anterior regions) having reached adult sizes in the youngest of our subjects. There were no significant effects of sex for any measures. These findings support anatomical studies indicating ongoing myelination of higher association areas throughout adolescence, but raise intriguing questions about anterior-posterior gradients of interhemispheric myelination.