The effect of treatment with bed rest alone was evaluated in 16 patients with high altitude pulmonary edema of mild to moderate severity at an altitude of 3,750 meters in the central Peruvian Andes. The results were compared with those in 20 patients who received conventional therapy including the continuous administration of oxygen and bed rest. A system of grading the severity of high altitude pulmonary edema based on clinical symptoms and signs, radiologic findings and heart rate and respiratory rate was developed. The severity of pulmonary edema as evaluated with the grading system was similar in the two groups of patients. Treatment with bed rest alone resulted in complete recovery in all patients over a mean period of 60 hours. No treatment failure occurred. Similar results were obtained with oxygen therapy combined with bed rest, except that the relief of symptoms was more rapid, the decrease in heart rate and respiratory rate was greater and the recovery period was slightly shorter. High altitude pulmonary edema of mild to moderate severity can be treated successfully with bed rest alone without the administration of oxygen and without moving the patient to a lower altitude. Oxygen therapy is more effective and when available should be used in all cases of high altitude pulmonary edema.