Patients with pulmonary arterial hypertension, but not portopulmonary hypertension will be excluded. Patients will be excluded from consideration if they do not have severe PTPH, if invasive hemodynamic assessment is contraindicated, or if there are any contraindications to sildenafil. Patients with unstable coronary syndromes or otherwise significant unrevascularized coronary artery disease and myocardial ischemia as determined by cardiac stress testing and/or coronary angiography will also be excluded. Moribund patients will not be considered for enrollment.