The status of long-term, outpatient oxygen therapy is reviewed, particularly as it is applied to patients with chronic obstructive pulmonary disease. Recent clinical trials have shown that in stable hypoxemic patients (arterial O2 tension less than 60 torr) with obstructive disease, survival is prolonged by chronic O2 therapy, and that the more continuous the therapy is, the better the survival rate. The mechanism of the improved survival rate is not clear, although O2 therapy decreases the hematocrit and pulmonary vascular resistance and may improve neuropsychological function. In such patients, nocturnal decreases in arterial O2 saturation are associated with carbon dioxide retention and may contribute to pulmonary hypertension. Nocturnal hypoxemia is eliminated by the usual continuous O2 therapy. The incidence of nocturnal hypoxemia without upper airway obstruction or daytime hypoxemia is not known but may be small. Nocturnal O2 treatment for such patients may be indicated, but further studies are needed. Oxygen therapy during exercise in patients who are not hypoxemic at rest should be done only when benefits cannot be attributed to a placebo effect of O2 therapy.