An editorial notes that this landmark study is the largest to date on use of long-term oxygen therapy. It suggests that on the basis of all available current data, long-term oxygen therapy should be prescribed to prolong survival among patients with COPD who have chronic (>3 weeks) severe resting hypoxemia (PaO2 of ≤55 mm Hg or SpO2 of <88%) while they are breathing ambient air. However, it should not be routinely prescribed in patients with mild or moderate hypoxemia at rest or during exercise. It adds that a trial of oxygen use might still be appropriate in selected patients with moderate exertional hypoxemia and intractable breathlessness despite appropriate evidence-based treatment.