2. Biologic studies: The purpose of these studies is to determine the effect of hydroxyurea therapy on blood and urine markers of the severity of sickle cell disease. Patients participating in these studies they will have one blood sample of 2 teaspoons (10 milliliters) and a urine sample collected before starting hydroxyurea therapy. Another blood and urine sample will be collected when they complete the study. These samples will be used to analyze markers associated with the following disease processes in sickle cell disease: