Patent ductus arteriosus is more likely to be found in patients born at high altitudes. The effect becomes manifest at altitudes of over 3,000 meters and is most evident at altitudes of over 4,000 meters. It is presumed that mechanical factors affecting the pulmonary circulation as well as the lower oxygen tension may influence persistent patency of the ductus arteriosus. High altitude does not appear to be an etiologic factor in coarctation of the aorta. © 1960.