Cardiac output response was examined to determine its relationship to exercise and arterial hypoxia, pulmonary hemodynamics, and the clinical/lung mechanics profile of patients with chronic obstructive pulmonary disease (COPD). Twenty patients with COPD were examined on the treadmill at rest and during steady state exercise at the highest workload comfortably tolerable to the subjects (60-70% VO2 max). In eight patients, the exercise cardiac output was less than 80% of predicted, while in the remainder (n = 12), the cardiac output response was normal (greater than 80% of predicted). It was concluded that hyperinflation and severe expiratory airflow limitation may be important determinants of the low exercise cardiac output response in some patients with COPD.