A case of identical (monozygotic) triplets, two of whom have agenesis of the corpus callosum, is presented. Prenatal ultrasonography and magnetic resonance imaging revealed two of the triplets to have agenesis of the corpus callosum and the third triplet to have an intact corpus callosum. No such case has been reported in the literature. Theories of the etiology and pathogenesis of corpus callosum agenesis are discussed. In this case, unequal X-inactivation as related to the twinning process is implicated as the cause of the agenesis.