Twenty-eight COPD patients underwent right heart catheterization while in clinically stable condition. Pulmonary vascular response to oxygen was evaluated by the percent change in pulmonary arteriolar resistance after 100% oxygen inhalation (% delta PAR), and its relation to the pressure-flow relationship during incremental exercise was assessed. Mean pulmonary arterial pressure (PPA) during exercise was plotted against the cardiac index (C.I.) from rest to maximal exercise in each patient. In most of the patients, the changes in PPA were nearly linear to the C.I. Therefore, a slope could be obtained from the regression equation in each patient. Patients were divided into two groups according to whether their % delta PAR was greater than 20 defined as a responder (RES), or less than five defined as a non-responder (N-RES). Seven out of 28 patients were RES, nine were N-RES, RES showed a higher %FEV1.0 level, C.I. and stroke volume index (S.I.) at maximal exercise, and a lower level of RV/TLC as well as slope. The slope correlated significantly with %DLCO (r = -0.724, p < 0.01), baseline PAR (r = 0.562, p < 0.01) and % delta PAR (r = -0.522, p < 0.01). These results suggest that the diminished pulmonary vascular bed, and the distensibility of pulmonary vessels, appear to contribute to the steepness of the slope and reduced % delta PAR in patients with COPD.