Background: Heart failure (HF) is frequently complicated by elevated pulmonary vascular resistance associated as a result of dysregulation of nitric oxide-mediated vascular smooth muscle tone. The resulting pulmonary hypertension directly affects right ventricular function and may affect exercise capacity, morbidity, and mortality. Sildenafil, a type 5 phosphodiesterase inhibitor, lowers pulmonary vascular resistance in pulmonary hypertension by increasing intracellular levels of the nitric oxide. The aim of the study was to evaluate the improvement of exercise performance, ventilatory efficiency, and pulmonary hypertension after 8 weeks of regular sildenafil use in outpatients with CHF. Methods and results: Fourty patients with controlled heart failure on standard antifailure treatment were enrolled in this study. Half of them received sildenafil 50 mg twice daily for 8 weeks and the other half was taken as a control group. Echo-Doppler, cardiopulmonary exercise testing, and clinical follow-up were done. There was a statistically significant drop of PAP in the sildenafil group from 58.4 ± 2 mmHg to 40.3± 0.5 mmHg and improvement in VO2 Peak, VE/VCO2 slope, T-1/2 Vo2 (min) and T-1/2 VCO2 (min) from 17.2± 2, 39.1 ± 6, 2.0± 0.5 and 2.0 ±0.4 to 20 ± 2.5, 42.1 ± 5, 1.9± 0.7 and 1.8± 0.2 respectively (p< 0.05). Seven patients of the sildenafil group (35%) showed improvement of functional class from NYHA class II to class I. Concerning cardiac events during follow up period, as regards active group, decompensated heart failure occurred in 1 patient (5%), ischemia occurred in 1 patient (5%), and arrhythmias occurred in 2 patients (10%), while in the control group, decompensated heart failure occurred in 7 patients (35%), ischemiaoccurred in 4 patients (20%), and arrhythmias occurred in 5 patients (25%), no CVA occurred in both groups. Conclusions: The present study showed that in patients with systolic HF, type 5 phosphodiesterase inhibition with sildenafil lowers the pulmonary artery pressure significantly and improved ventilatory efficiency monitored by VE/VCO2 slope, oxygen uptake kinetics and also improved exercise performance.