In each of the 2 cases of monozygotic twins reported here, 1 of the fetuses was suspected, as a result of a prenatal ultrasonographic examination, of having congenital heart disease (CHD). One of twins (Case 1) had a hypoplastic left heart (HPLH), coarctation of the aorta (CoA), mitral stenosis (MS), and an atrial septal defect (ASD). Of the other twins (Case 2), 1 fetus had pure pulmonary atresia (PPA) and tricuspid regurgitation (TR). In Case 1, the fetus with CHD had a single umbilical artery, the other had otherwise. In Case 2, the fetus with CHD had a velamentous insertion and an accessory placenta, and the other fetus had a marginal insertion. These findings suggest that environmental factors such as blood flow, blood pressure and so on have a role in the etiology of CHD.