Applying in vivo magnetic resonance (MR) morphometry in healthy adults we have recently discovered that the relationship between forebrain volume (FBV) and the midsagittal size of the corpus callosum (CC) follows a geometrical rule according to which larger brains have a relatively smaller midsagittal CC. This allometric relation was taken as support for the hypotheses of Ringo and co-workers suggesting that brain size may be an important factor influencing interhemispheric connectivity and lateralization. In this paper we examined whether the aforementioned relation between FBV and CC size also holds for healthy children between 3 and 14 years of age. We confirmed this relationship as previously found for adults. Thus, the geometrical rule and the implications associated with it apply for a wide age range. In addition we found significant correlations with age for posterior and mid-parts of the CC even when FBV was controlled for, suggesting an anterior to posterior maturation gradient of CC development.