Earlier studies had suggested that the addition of erythropoietin (Erythropoietin) therapy to chronic hydroxyurea therapy may induce fetal hemoglobin at higher, more widely distributed, levels. We plan to test this in patients with sickle cell disease who have chronic kidney disease, which, presumably, leaves them with a depressed Erythropoietin reserve and an inability to tolerate standard doses of F-inducing therapy with hydroxyurea, and in patients with pulmonary HTN, which carries an ominous prognosis in SCD. A secondary endpoint of this study will be to evaluate if hydroxyurea plus Erythropoietin therapy can improve cardiovascular aerobic capacity in general, and in particular minimize symptoms and morbidity in patients with both chronic kidney disease and pulmonary HTN.