The Nocturnal Oxygen Therapy Trial, a study of oxygen therapy in patients with chronic obstructive pulmonary disease, has combined the experience of six centers concerning the selection of patients for oxygen treatment. Forty-five percent of hypoxic patients initially selected for the study improved enough during the one month of outpatient observation to allow suspension of plans to treat them with oxygen. Therefore, long-term oxygen therapy plans should only be made without a month of careful observation. Resting Pao2 values less than 40 mm Hg suggest instability or that chronic obstructive pulmonary disease is not the only cause of the hypoxemia. Nasal prong oxygen flow of 1 to 3 L/min reversed hypoxemia in 95% of stable patients with chronic obstructive pulmonary disease.