It is well known that exercise limitation frequently observed in patients with CHF is multifactorial and includes abnormal ventilatory responses.20The 6MWT has been used to evaluate functional capacity in symptomatic CHF patients.21–22 In a recent systematic review, Olsson et al23 concluded that although there is uncertainty about the utility of this test for the evaluation of pharmacologic therapy, the 6MWT may be of greater value in patients with advanced CHF, in whom it may function as a maximal exercise test. A possible disadvantage of using the 6MWT as an outcome measure in studies of CHF treatment is that the verbal encouragement of the person administering the test could affect the distance walked by the patient.23 The advantages of the 6MWT rather than the bicycle or treadmill exercise are that the test is simple, inexpensive, and easily performed; a more natural form of exercise that may better reflect daily activity; and has important prognostic information.21–23 In this study, the physical limitation in patients with CHF was confirmed through the 6MWT, observing that most patients showed muscle weakness and dyspnea during the test and hardly completed the test. It is reasonable to assume that the improvement of the pulmonary function and associated symptoms caused by the CPAP therapy contributed to the progressive increase in the distance walked in the 6MWT by patients with CHF in our study. However, we may consider that other investigators have shown that the improvement of exercise tolerance in CHF patients could also be due to the beneficial effects of CPAP therapy on other parameters. It has been shown that CPAP application causes significant improvements in cardiac function of CHF patients, including reduction in left ventricular preload24and afterload,25augmented left ventricular ejection fraction,26 reduction in mitral regurgitant fraction,26and in myocardial oxygen consumption plus carbon dioxide production.27 These improvements in cardiovascular function could be as a consequence of the CPAP-induced increase in intrathoracic pressure.24–26,28 Other beneficial cardiovascular effects of CPAP on CHF patients have been reported, including inhibition of cardiac sympathetic nervous activity,27 and reduction in plasma atrial natriuretic peptide concentration.26 It is interesting that in this study, we observed a substantial improvement in pulmonary function and 6MWT distance even after short-term CPAP therapy. Our findings are in agreement with others24,27,29 showing beneficial effects of short-term CPAP therapy in CHF patients. Based on other studies,27,29–30 we speculate that among possible mechanisms could account for some of the favorable effects of short-term CPAP on pulmonary function and 6MWT distance in CHF patients are decreases of cardiac sympathetic nervous activity,30 myocardial work and oxygen consumption,27 mitral regurgitation,29 and improvement of ejection fraction.29 Although the patients were randomly allocated in the CPAP or control groups, we cannot rule out the possibility that the pretreatment value of the left ventricular ejection fraction 8% higher in the CPAP group could additionally contributed to the improvement of cardiopulmonary function and tolerance to physical exercise in this group.