This was an open label assessment of hydroxyurea pharmacokinetics and relative bioavailability conducted in two independent cohorts of pediatric patients with sickle cell anemia or sickle beta-zero thalassemia. A total of 42 participant were enrolled, out of which 39 were enrolled and dosed. A one-compartment population PK model was successfully developed to describe the time course of hydroxyurea concentrations after oral administration in sickle cell anemia participants from both study arms. The relative bioavailability analysis showed very similar pharmacokinetics for peroral liquid and capsule formulations. No adverse events (AEs) were classified as being caused by the study drug, and no AEs led to study drug interruption. Lastly, no significant changes from baseline in laboratory values, vital signs, or physical examination results were found. Hydroxyurea was well tolerated as evidenced by safety monitoring and reporting.