 Patients with sickle cell disease have abnormal hemoglobin the protein in red blood cells that carries oxygen to the body. This abnormality causes red blood cells to take on a sickle shape, producing disease symptoms. Fetal hemoglobin, a type of hemoglobin present in fetuses and babies, can prevent red cells from sickling. The drug hydroxyurea increases fetal hemoglobin production in patients with sickle cell disease by making a molecule called nitric oxide. The drugs L-arginine and Sildenafil Viagra increase the amount or the effect of nitric oxide. This study will evaluate: 