 There is now ample clinical evidence that hydroxyurea is a safe and effective medication for adults and children with sickle cell anemia SCA, and most hematologists agree the short-term safety and efficacy of hydroxyurea has been proven. The National Heart, Lung, and Blood Institute have recently released evidence-based guidelines for SCA, recommending that hydroxyurea be offered to all affected children as young as nine months of age, regardless of clinical severity. Despite the overwhelming evidence demonstrating safety and efficacy, hydroxyurea remains underutilized for a variety of reasons. In this prospective study, the investigators will utilize innovative strategies designed to address and overcome some of the barriers that currently limit the use of hydroxyurea for children with SCA. The investigators will utilize novel laboratory techniques and pharmacometric modeling in order to accurately predict the most effective hydroxyurea dose referred to as the maximum tolerated dose. The investigators aim to develop a screening urine test to objectively and accurately determine adherence to hydroxyurea therapy. In addition, the study will document critical laboratory and clinical characteristics of this unique population of patients with SCA who begin hydroxyurea at a young age. 