Measurement of O2 (VO2) consumption makes it possible to refine non-invasive assessment of congestive heart failure (CHF). The peak of VO2 during exercise is determined by the maximum values of cardiac output and of the arteriovenous O2 difference. It thus provides semi-quantitative hemodynamic information. In addition, reproducibility of the VO2 peak is quite satisfactory in these patients; it allows them to be classified according to their VO2 peak and permits more objective assessment of therapeutic management. Finally, the anaerobic threshold, when it can be determined, provides the same type of information during sub-maximum exercise.