Cardiac catheterization data from eight patients with severe chronic obstructive lung disease and pulmonary hypertension at rest (greater than 25 mm Hg) were compared with those obtained from 14 patients with mild to moderate disease whose pulmonary artery pressure was within the normal range at rest (mean 15 (SEM 1) mm Hg), but increased with exercise (30 (2) mm Hg). We obtained lung sections from necropsy material from the group with severe disease, and from surgical specimens in the group with mild to moderate disease, and compared the structure of the vasculature in these groups with that obtained from surgical specimens in a non-smoking control group of seven patients. Oxygen administration either at rest or during exercise did not greatly affect the pulmonary arterial pressures. When cardiac index was plotted against pulmonary artery pressure at rest and during exercise and extrapolated to the axis there was no evidence for a critical closing pressure in either group. The vessels in the groups with mild to moderate and severe chronic obstructive lung disease showed intimal thickening (each 19% (SD 0.5%)) by comparison with the non-smoking group (16% (0.5%]. The group with severe disease, in addition, had medial hypertrophy (27% (0.5%) versus 24% (SD 1%) in the non-smoking group). These data are consistent with the idea that the diseased vessels are distorted and rigid. The lack of effect of breathing oxygen on the vascular response at rest and during exercise suggests that hypoxic vasoconstriction has a minimal role in the pulmonary hypertension of chronic obstructive lung disease. The data suggest that the intimal changes could narrow the vessel calibre in those patients with mild to moderate disease, and that the thickened media present in the vessels from patients with severe disease may act in concert with the enlarged intima to produce more severe vascular obstruction.