T HE transition from the fetal to the neonatal state necessitates marked ad- justments in cardiovascular dynamics as the lungs become air filled. Chief among these changes is a decrease in pulmonary vascular resistance and increased pulmon- ary blood flow with an associated decrease in pulmonary artery pressure.2 These changes, however, do not occur immedi- ately upon aeration of the lungs; during the first 2 weeks of life the main pulmonary artery pressure remains considerably higher than in older children.5 Persistent pulmo- nary hypertension in the newborn is usually associated with pulmonary or congenital heart disease, but, rarely, fetal circulatory dynamics may persist in the absence of structural cardiovascular or pulmonary disease. These infants are cyanotic and have marked elevation of pulmonary ar- tery pressures."7 Four patients whose clinical findings are consistent with the syndrome of "persistent pulmonary vascular obstruction" have re- cently been treated in the intensive care nursery of The Montreal Children's Hos- pital and their clinical and roentgenologic findings are presented below. Their clinical and laboratory data are shown in Table I. REPORT OF CASES