Long-term oxygen is the only therapy that has been shown to improve survival in patients with chronic obstructive pulmonary disease. The aim of this study was to assess the predictors of survival in such patients treated with long-term oxygen therapy. We studied 179 patients who were assessed for long-term oxygen therapy in two Departments of Respiratory Medicine: in Warsaw and in Edinburgh. Those who died following the prescription of long-term oxygen therapy had a similar forced expiratory volume in the first second (FEV1) and arterial carbon dioxide tension, but a slightly lower arterial oxygen tensions (p less than 0.05) than those who survived (p less than 0.05). A small but significant fall in FEV1 and a rise in arterial carbon dioxide tension (p less than 0.05) occurred in both survivors and nonsurvivors after treatment with oxygen, but arterial oxygen tension breathing air continued to fall only in those who died (p less than 0.005). Only two variables were independent predictors of survival in patients with chronic obstructive pulmonary disease treated with long-term oxygen therapy. These were the arterial oxygen tension and the mean pulmonary arterial pressure (Ppa). However, when the calculation was made on patients with PaO2 less than or equal to 60 mm Hg (n = 154), then FEV1 and PaO2 but not Ppa were found to predict survival.