The study will analyze demographics (age and gender), origin, genotype, clinical phenotype (vaso-occlusive or hemolytic), transfusion history (including exchange), and folic acid use, average laboratory values up to three years pre-hydroxyurea and for the period post-hydroxyurea therapy including total hemoglobin level, fetal hemoglobin level, hemoglobin S level, white blood count, platelet count, lactate dehydrogenase level, total and direct bilirubin levels, aspartate and alanine aminotransferase levels, and serum creatinine level.