During exercise patients with chronic left heart failure ventilate more than normal individuals at the same workload; the ratio of minute ventilation to minute production of carbon dioxide (VE/VCO2) is increased. The relation between increased VE/VCO2, severity of heart failure, and exercise capacity has not been defined. VE/VCO2 was measured in 47 patients with chronic left heart failure (New York Heart Association grades II and III) and in 1009 healthy controls. Exercise capacity was assessed by peak oxygen consumption (VO2max) during progressive exercise. In the controls VO2max ranged from 25 to 93 ml/kg/min; VE/VCO2 was 17-36 and did not correlate with VO2max. In chronic left heart failure the VO2max ranged from 9 to 29 ml/kg/min; VE/VCO2 was 22-42 and correlated strongly with VO2max. End tidal carbon dioxide and respiratory rate at peak exercise were similar in the controls and patients with chronic left heart failure. The increase in VE/VCO2 on exercise in chronic left heart failure indicates increased physiological dead space, presumably caused by a ventilation-perfusion mismatch. In the controls and patients with chronic left heart failure the relation of VE/VCO2 to VO2max was curvilinear with a threshold of VO2max below which VE/VCO2 started to rise above the normal range. This point of inflection may be explained by the existence of a critical level of cardiac function necessary to perfuse adequately all lung zones on exercise.