Drug Distribution Center - Treatment of Pulmonary Hypertension and Sickle Cell Disease with Sildenafil Therapy (WalkPHaSST) Approximately 1,000 patients with sickle cell disease will be screened for pulmonary hypertension (PH) by echocardiography. 132 subjects, age 12 or older, with mild or moderate to severe PH will be randomized to a double-blinded, randomized, placebo-controlled clinical trial designed to test the effects of 16 weeks of sildenafil therapy on exercise capacity (six minute walk distance) and pulmonary artery pressure. Subjects with moderate to severe PH will undergo right heart catheterization prior to randomization. The contractor serves as a Data Coordinating Center (DCC). Ten Clinical Sites and a pharmacy distribution center have also been awarded. Public Health Relevance: