 The primary objectives of this prospective study of hydroxyurea for children with sickle cell anemia are 1 Develop and prospectively evaluate a population pharmacokineticpharmacodynamics model to predict the maximum tolerated dose MTD; 2 Identify urine biomarkers of hydroxyurea adherence using a novel metabolomics approach; 3 Identify pharmacogenomics modifiers of hydroxyurea MTD; and 4 Longitudinal monitoring of the effect of hydroxyurea upon organ function and quality of life. 