Continuous positive airway pressure (CPAP) is an effective treatment for obstructive sleep apnea. It is known, that there are beneficial effects on cardiac function, which might be explained by suppression of apnea and specific hemodynamic effects of CPAP. Therefore, CPAP might act as an adjunct therapy in heart failure, even in the absence of sleep apnea. In the present study, 11 patients with congestive heart failure (EF=23.1+/-6.9%) without sleep apnea (AHI 3.0+/-1.2/h) were treated with nocturnal CPAP. Cardiopulmonary exercise testing was performed at baseline and after 8.6 +/-1.3 months. All patients underwent heart catheterization and myocardial biopsy to exclude myocarditis at baseline. Five (46%) of the 11 patients did not complete the study because of poor compliance and irregular use of the CPAP device. Six (54%) of the patients used CPAP regularly (>6 h/night) and completed the study. Cardiopulmonary exercise testing showed an improvement of work load (96+/-36 Watt vs. 112+/-34 Watt; P=0.025) and VO2 peak (1227+/-443 ml vs. 1525+/-470 ml; P=0.01). Oxygen-pulse was increased, although that did not reach significance (11.2+/-4.8 ml/beat vs. 12.6+/-3.9 ml/beat). In conclusion, CPAP might have beneficial effects on exercise capacity in patients with congestive heart failure even in the absence of sleep apnea. Nevertheless, poor compliance seems to be a limiting factor.