Progressive pulmonary hypertension associated with cyanosis was encountered in two infants. In the first infant, at 45 days of age, pulmonary arterial pressure became normal with oxygen breathing and cyanosis was abolished. Later, cyanosis became increasingly unresponsive to oxygen and death occurred at 70 days of age. The second infant maintained pulmonary arterial hypertension while breathing oxygen; however, a right-to-left ductal shunt was reversed during oxygen breathing apparently due to an increase in systemic blood pressure and a favorable alteration in the systemic: pulmonary vascular resistance ratio. This infant also showed progressive cyanosis unresponsive to oxygen, and died at the age of 62 days. The etiology could not be established before or after death in either infant. The hemodynamic and autopsy findings suggested the persistence of some of the features of the fetal pulmonary physiology.