Autopsy findings and a morphometric study of the lungs were compared in 18 subjects receiving nocturnal oxygen and 15 receiving continuous oxygen in the National Heart, Lung, and Blood Institute Nocturnal Oxygen Therapy Trial (about half of those who died). The emphysema score, average interalveolar wall distance, central airway lesions, peripheral airway lesions, and the ratio of weights of left ventricle plus septum to right ventricle were similar in the two groups. The causes of death in the two groups were also similar. This evidence supports the hypothesis that the improved prognosis observed with continuous oxygen therapy nocturnal oxygen therapy in patients with severe chronic airflow obstruction and hypoxemia was due to treatment. There was a trend for there to be more interstitial fibrosis and type 2 alveolar epithelial cell hyperplasia in those treated with nocturnal oxygen; in the hands of one observer, the type 2 cell hyperplasia was significant.