Monochorionic diamniotic (MCDA) twins identified by ultrasound in early pregnancy are rarely dizygotic. Since Shouter et al. first reported that MCDA twin pregnancies can occur in dizygous (DZ) twin pregnancies, there have been about ten reports of such cases [1]. MCDZ twins can be easily overlooked except in cases of discordant sex discovered during prenatal care or delivery of MCDA twins. In our case, chimerism of a set of MCDZ twins was confirmed by STR analysis for both parents and the live male twin after the discovery of discordant sex in a pair of MCDA twins at delivery. Testicular atrophy was suspected in a follow-up sonogram of the testes of the live male twin; Testicular hypoplasia was revealed by a subsequent testicular biopsy. We report herein a rare case of testicular hypoplasia in a male infant with chimerism that was observed on follow-up examination after birth.