The prognosis of transposition of the great arteries improved tremendously with the development of an early medico-surgical strategy including balloon atrioseptostomy, prostaglandin infusion and the arterial switch operation within the first days of life. Nevertheless, some patients still die preoperatively. We report on two newborn infants whose fatal outcome was promoted by an inadequate intercirculatory mixing. Since the diagnosis was not immediately made, the restrictive foramen ovale resulted very quickly in deep metabolic acidosis and balloon atrioseptostomy performed yet in the first hours of life could not prevent death. We emphasize the importance of prenatal echographic detection of this defect, only way to plan a balloon septostomy immediately after delivery in those infants suffering from inadequate atrial mixing.