Hydroxyurea therapy reduces hemolysis and decreases serum bilirubin levels in children and adults with sickle cell anemia (SCA) and may therefore help prevent the development of cholelithiasis in this patient population. We recently reported that a promoter polymorphism in the uridine diphosphoglucuronate glucuronosyltransferase 1A (UGT1A) gene affects steady-state bilirubin levels and the incidence of gallstones in children with SCA. We have now analyzed the influence of the UGT1A genotype on the therapeutic response to hydroxyurea. A large cohort of children with SCA taking hydroxyurea therapy at the maximum tolerated dose demonstrated significant reductions in hemolysis independent of UGT1A promoter polymorphism genotype, but the hydroxyurea-related decreases in serum bilirubin levels were significantly different. Children with the wild-type 6/6 UGT1A genotype demonstrated normalized bilirubin levels with hydroxyurea therapy, but children with the heterozygous 6/7 or abnormal 7/7 genotypes did not. Children with the abnormal 7/7 genotype, which confers the phenotype of Gilbert syndrome, had bilirubin levels greater than 3 mg/dL despite full-dose hydroxyurea therapy. These data indicate the UGT1A promoter polymorphism is a powerful nonglobin genetic modifier in SCA that influences serum bilirubin both at baseline and on hydroxyurea therapy. UGT1A promoter polymorphisms may therefore influence the ability of hydroxyurea to prevent gallstone formation in patients with SCA.