This is an effort to screen approximately 1,000 sickle cell disease patients for pulmonary hypertension by echocardiography followed by a double-blinded, randomized, placebo-controlled clinical trial designed to test the effects of 16 weeks of chronic sildenafil therapy on exercise endurance (six minute walk distance) and pulmonary artery pressure in 132 patients age 12 or older with pulmonary hypertension (PH) and sickle cell disease (SCD). The contractor shall serve as a Clinical Site for the Phase II/III Trial of Sildenafil for Sickle Cell Disease-Associated Pulmonary Hypertension. A Data Coordinating Center (DCC), eight Clinical Sites, and a pharmacy distribution center are being awarded.