Fetal hemoglobin (Hb F) levels during extended hydroxyurea therapy in infants with SCA. The upper curve (♦) represents the study patients receiving extended hydroxyurea therapy (mean ± SD values for Hb F), while the bottom 2 curves represent the expected values for untreated American²⁷  (▪) and Jamaican children³⁰  (▴) with SCA. Average fetal hemoglobin levels remain close to 20% during extended hydroxyurea therapy.