Cardio-pulmonary exercise testing was performed in 99 normal subjects and 382 patients with cardiac disease in order to evaluate anaerobic threshold (AT) and related parameters as indices for assessing the severity of heart failure. AT could be determined easily during ergometer exercise testing with ramp protocol by monitoring minute ventilation (VE), oxygen uptake (VO2) and carbon dioxide output (VCO2). Peak VO2 and the ratio of VO2 rising to work rate increment (delta VO2/delta WR) were also determined. There was good correlation between the AT determined by respiratory measurement and that determined by arterial lactic acid concentration (r = 0.93, n = 15). The reproducibility of AT was excellent between 2 testings with a 3-hour interval. AT (ml/min/kg) and peak VO2 (ml/min/kg) declined with age, and males showed higher values than females in both indices. %AT, determined by the predicted AT values of each age and sex, decreased as NYHA class progressed as follows: 90.2 +/- 15.4% in class I, 76.9 +/- 13.8% in class II, and 59.7 +/- 11.9% in class III. Although delta VO2/delta WR was not influenced by age or sex, it also decreased as the severity of heart disease progressed. These results suggest that indices from cardiopulmonary exercise testing, especially AT, are closely related to the pathophysiology of heart failure, so that they are objective and reliable parameters for evaluation of the severity of heart failure and are sensitive enough to detect the efficacy of therapeutic intervention for heart failure.