This study will examine the use of hydroxyurea and erythropoietin for treating sickle cell disease in patients who also have kidney disease or pulmonary hypertension (high blood pressure in the lungs). Hydroxyurea increases production of fetal hemoglobin in the red blood cells of patients with sickle cell disease, reducing the amount of sickle cells that cause pain and other complications requiring hospitalizations. However, hydroxyurea treatment has limitations: patients with sickle cell disease who have developed kidney disease may not be able to get the full benefit of the medicine, and hydroxyurea alone may not be able to treat life-threatening complications such as pulmonary hypertension or stroke. This study will determine which of two dosing schedules of hydroxyurea and erythropoietin is more effective for treating patients with sickle cell disease who also have kidney disease or pulmonary hypertension, and will examine whether the two drugs can lower blood pressure in the lungs.