Background— Pulmonary vascular (PV) distensibility, defined as the percent increase in pulmonary vessel diameter per mm Hg increase in pressure, permits the pulmonary vessels to increase in size to accommodate increased blood flow. We hypothesized that PV distensibility is abnormally low in patients with heart failure (HF) and serves as an important determinant of right ventricular performance and exercise capacity. Methods and Results— Patients with HF with preserved ejection fraction (n=48), HF with reduced ejection fraction (n=55), pulmonary arterial hypertension without left heart failure (n=18), and control subjects (n=30) underwent cardiopulmonary exercise testing with invasive hemodynamic monitoring and first-pass radionuclide ventriculography. PV distensibility was derived from 1257 matched measurements (mean±SD, 8.3±2.8 per subject) of pulmonary arterial pressure, pulmonary arterial wedge pressure and cardiac output. PV distensibility was lowest in the pulmonary arterial hypertension group (0.40±0.24% per mm Hg) and intermediate in the HF with preserved ejection fraction and HF with reduced ejection fraction groups (0.92±0.39 and 0.84±0.33% per mm Hg, respectively) compared to the control group (1.39±0.32% per mm Hg, P <0.0001 for all three). PV distensibility was associated with change in right ventricular ejection fraction (RVEF, ρ=0.39, P <0.0001) with exercise and was an independent predictor of peak VO _2. PV distensibility also predicted cardiovascular mortality independent of peak VO ₂ in HF patients (n=103; Cox hazard ratio, 0.30; 95% confidence interval, 0.10–0.93; P =0.036). In a subset of patients with HF with reduced ejection fraction (n=26), 12 weeks of treatment with the pulmonary vasodilator sildenafil or placebo led to a 24.6% increase in PV distensibility ( P =0.015) in the sildenafil group only. Conclusions— PV distensibility is reduced in patients with HF and pulmonary arterial hypertension and is closely related to RV systolic function during exercise, maximal exercise capacity, and survival. Furthermore, PV distensibility is modifiable with selective pulmonary vasodilator therapy and may represent an important target for therapy in selected HF patients with pulmonary hypertension. Clinical Trial Registration— URL: http://www.clinicaltrials.gov . Unique identifier: NCT00309790.