Twenty-seven cases of pulmonary edema of high altitude are reported. The disease occurred at an altitude of 3,000 meters (9,900 feet) or higher. Pulmonary edema of high altitude was observed in persons with normal hearts and often in young subjects. It occurred during the first trip, or in one or more of the several trips, made to the higher altitude. Several patients were symptom free in previous and subsequent trips to the same altitude. The disease was often observed in normal subjects adapted to the altitude going back to the highlands after spending a variable length of time at sea level. Respiratory infections were an aggravating or precipitating factor. Pulmonary edema of high altitude was usually preceded by symptoms which, as a rule, appeared shortly after arrival at the high elevation. The knowledge of the premonitory symptoms is important to prevent death. The treatment of choice is oxygen or, when possible, descent to lower altitudes or sea level. The cause of pulmonary edema of high altitude is obscure. The various possible mechanisms are briefly discussed. © 1961.