This study evaluated the relationship between the oxygen consumption (VO2) and cardiac output and heart rate during progressive exercise in the upright position in 26 patients with severe chronic obstructive pulmonary disease. Forced expiratory volume in one second (FEV1) was 0.82 +/- 0.21 L, and single-breath carbon monoxide diffusing capacity was 39 +/- 20% predicted. Cardiac outputs were measured by the direct Fick method. The patients as a group had a normal cardiac output for the level of VO2. The mean pulmonary artery pressure in our patients (22.5 +/- 10.1 mmHg) was increased at rest; during exercise, it increased abnormally to 45.5 +/- 18.9 mmHg. The heart rates were increased both at rest and during exercise, and the increase in heart rate for an increase in VO2 was higher than normal. The relative tachycardia observed was probably related to a combination of abnormal arterial blood gases, concomitant bronchodilator administration, deconditioning, and right ventricular dysfunction. The relative tachycardia did not appear to have an adverse effect on exercise tolerance because the ratio of maximal exercise ventilation to the FEV1 exceeded 35 in those patients with observed maximal heart rates above 90% of predicted. The results of this study suggest that improvements in the exercise tolerance of these patients is dependent upon improving their ventilatory capabilities or the efficacy of their ventilation.