OBJECTIVES: To assess adherence to hydroxyurea therapy in children with sickle cell anemia (SCA), evaluate the association between adherence and hematologic profile, and identify barriers and facilitators of adherence. STUDY DESIGN: Children with SCA (n=75) receiving hydroxyurea were recruited for a single-institution cross-sectional study. The primary outcome was association between treatment adherence and percent fetal hemoglobin (HbF). RESULTS: Good adherence was estimated at 82% with visual analog scale, 84% with Morisky score, 85% with medical provider report, 77% with clinic visits, and 49% on the basis of pharmacy refills. Increase in HbF was moderately associated with good adherence as measured with the parent/proxy Morisky score (r=-0.39; 95% CI, -0.58-0.17; P < .01) and prescription refills (r=0.39; 95% CI, 0.16-0.57; P < .01). The number of pharmacy refills and the Morisky score explained 23% of the variation in HbF response. CONCLUSIONS: Adherence was > or =75% with 4 of 5 measures. Pharmacy refills and the Modified Morisky Scale may be used to identify children at high risk for poor response because of non-adherence and children with good adherence with poor response because of individual pharmacodynamics. Future research should prospectively compare adherence measures and evaluate methods to improve treatment adherence.