Cardiac catheterization studies have been carried out in 2 young male subjects who experienced acute pulmonary edema after a brief sojourn at sea level, when they returned to their native town located at 14,200 feet above sea level. The investigation was performed at this altitude during the acute episode and was repeated after complete recovery. Further studies were made in 1 subject after prolonged residence at sea level. Severe hypoxemia, a marked degree of pulmonary hypertension and increased pulmonary vascular resistance were found during the acute pulmonary edema. These findings were associated with low cardiac output and pulmonary wedge pressure. The degree of pulmonary hypertension was significantly reduced after inhalation of 100 per cent oxygen. Following recovery, physiologic observations were similar to those seen in healthy residents well acclimatized to high altitudes. The data obtained suggest the presence of arteriolar constriction at the precapillary level due to severe hypoxia. Several factors may explain the severe hypoxemia observed in patients with high altitude pulmonary edema. Exercise and sleeping may decrease significantly the arterial oxygen saturation at high altitudes. Pulmonary edema occurred during exercise in our first case, and during sleep in the second. However, special susceptibility appears to be present in humans who develop pulmonary edema after rapid exposure to high altitudes. The role of a sudden rise of pulmonary artery pressure in producing pulmonary edema without elevation of pulmonary wedge pressure is not clear. © 1969.