Peak oxygen consumption is of great importance for the decision of heart transplantation in congestive heart failure. Moreover, the level of exercise capacity seems to depend on the etiology of congestive heart failure. This study compared 14 heart failure patients with idiopathic dilated cardiomyopathy (group 1) to 14 heart failure patients with cardiomyopathy due to ischemic heart disease (group 2), matched for sex (13 male, one female in each group), age +/-10 years, left ventricular ejection fraction +/-5% and pulmonary artery mean pressure +/-5 mm Hg, to assess exercise capacity and oxygen consumption independently of the age, sex and the level of left ventricular dysfunction. Right ventricular function was also assessed. No difference existed in terms of right ventricular parameters. Maximal exercise parameters were significantly higher in group 1 than in group 2. Peak oxygen consumption was statistically higher in group 1 than in group 2. In the whole population, a significant correlation was found between peak oxygen consumption and right ventricular ejection fraction (r=0. 44, P<0.02) but not between peak oxygen consumption and left ventricular ejection fraction. For similar levels of left ventricular dysfunction, exercise capacity and oxygen consumption appear to be better in idiopathic dilated cardiomyopathy than in ischemic cardiomyopathy, thereby suggesting that functional tolerance of left ventricular dysfunction might depend on the etiology of severe congestive heart failure.