The effects of long-term continuous O2 therapy on chronically hypoxemic patients are well established. Such therapy remains the most important means of improving survival and quality of life of these patients. The mechanism by which O2 induces these beneficial effects is not well understood, and several factors probably are involved. Patients who show stable hypoxemia are candidates for long-term continuous O2 therapy. O2 should be administered at least 19 hours per day, including sleep hours, and in a dose sufficient to raise arterial PO2 to 65-80 mm Hg. Continuous O2 therapy virtually eliminates hypoxemic episodes during sleep or exercise. Nasal prongs are the usual method for delivering O2 on long-term basis, and portable O2 supply systems permit patients to use O2 continuously without significant restriction of their activities.