Patients with chronic heart failure were evaluated simultaneously by conventional methods and by heart-lung exploration during exercise with the purpose of validating the latter method. Only 50 patients were evaluated on two occasions and therefore included in the correlative statistical study. In this study such data as NYHA classification, cardiothoracic ratio, ejection fraction, echocardiographic fibre shortening fraction and biochemical values (venous lactate, plasma noradrenaline levels) were compared with data obtained from evaluation during exercise, i.e. VO2 max, VO2 peak, anaerobic threshold, oxygen pulse, VCO2, O2 equivalent and respiratory quotient, alveolar ventilation per minute, duration and load of exercise. Very good correlation was found with the indices obtained from VO2 and in particular with the O2 pulse and the VO2 max percentage. Good correlation was also found with VCO2 and the O2 equivalent as well as with alveolar ventilation per minute. In contrast, the anaerobic threshold and respiratory quotient correlated poorly with the first set of data and therefore were disappointing. It appears from this study that the indices obtained from VO2 are highly representative of the heart-lung unit in patients with heart failure. It seems, however, that the muscular status of the patients plays a considerable role in their exercise capacity. We suggest that these indices should be used in clinical pharmacology studies.