To evaluate the severity of heart failure, cardiopulmonary exercise testing was performed in 99 normal subjects and in 382 patients with cardiac disease. Anaerobic threshold, peak oxygen uptake, and the ratio of increase in oxygen uptake to work-rate increment were determined by ventilatory and gas exchange measurements as indexes for assessing the severity of heart failure. Anaerobic threshold and peak oxygen uptake declined with age, and males showed higher values than females in both indexes. Anaerobic threshold, as the percentage of predicted value for age, sex, and body weight, decreased as New York Heart Association (NYHA) class increased as follows: 90.2 +/- 15.4% in class I, 76.9 +/- 13.8% in class II, and 59.7 +/- 11.9% in class III (mean +/- SD). Although the ratio of increase in oxygen uptake to work-rate increment was not influenced by age or sex, it decreased as the severity of heart disease progressed. These results suggest that indexes from cardiopulmonary exercise testing are closely related to the pathophysiology of heart failure. We conclude that these indexes can be used as objective and reliable parameters for evaluation of the severity of heart failure in terms of functional capacity.