A pathophysiological study of the pulmonary vasculature in 10 patients with hypoxic cor pulmonale and severe airways obstruction (five treated and five untreated with long-term oxygen) is presented. The media of muscular pulmonary arteries was normal or atrophic but, in the intima, there was active deposition of longitudinal muscle, fibrosis and elastosis. In the arterioles a medical coat of circular smooth muscle bounded by a new internal elastic lamina had developed, while there was deposition of longitudinal muscle and fibrosis in the intima. In five cases the lumen was subdivided into parallel tubes, found by serial section to lead into alveolar capillaries. These features are distinctive of hypoxaemia and obstructive airways disease. Changes continued until death. The conspicuous longitudinal muscle may be attributable to stretching of vessels round distorted terminal airways; further exploration into mechanisms is required. The hypothesis that vascular changes follow hypoxic vasoconstriction is no longer tenable. No correlations were found between quantitative pathological findings and arterial blood gas tensions, pulmonary artery pressure or haematocrit. There were no differences between patients treated or not treated with oxygen which might suggest that it arrests pathological changes. Thus, once a patient is given oxygen, survival probably depends as much on progressive mechanical changes in the lung as on continuing hypoxaemia.