 This study will follow two groups of patients. The first group, referred to as the New Cohort, will include mostly young infants who are not receiving hydroxyurea therapy upon entering the study. The starting dose of hydroxyurea for each of the participants in the New Cohort will be individually determined using the novel population PKPD dose-prediction model. The second group of study participants, referred to as the Old Cohort, will include patients who are already receiving hydroxyurea therapy upon study entry. Both the Old and New Cohort New Cohort will be included in the development of a urine biomarker of adherence and will be followed throughout the study to document the effect hydroxyurea has upon organ function and quality of life. It is important to note that this is not a therapeutic drug trial. Prior to enrollment in the study, participants, along with their families and clinical providers, have decided to initiate hydroxyurea therapy for clinical indications. Except for the dose prediction model for the New Cohort, participants will be treated and monitored according to the routine clinical practice guidelines of the Cincinnati Childrens Hospital Comprehensive Sickle Cell Center. 