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Purpose: Hydroxyurea therapy reduces clinical complications in sickle cell disease. While evaluating the clinical and laboratory responses to hydroxyurea in children with sickle cell disease, we concurrently objectively monitored, for the first time during such treatment, compliance with therapy. Because most deaths in affected children are related to infection, we also evaluated the impact of hydroxyurea on splenic function, estimated by the percentage of red cells containing endocytic vacuoles (“pitted” cells) over 1 year of therapy.Patients and Methods: Seventeen children with a history of ≥3 hospital admissions in the previous year, aged (mean ± standard error of mean) 12.3 ± 1.2 years, were treated with hydroxyurea. Clinical and laboratory assessments monitored efficacy and toxicity. Compliance was monitored using computerized pill bottles containing cap microprocessors which monitor the frequency of bottle openings.Results: Over 18.5 ± 2.1 months, compliance with hydroxyurea (as determined by percent of the perscribed drug actually taken) was 96 ± 2%, resulting in increases in mean fetal hemoglobin from 7.7 ± 1.6% to 16.7 ± 1.8% (p <0.005). In 11 patients who reached maximum tolerated doses, an increase to 18.8 ± 2.5% (p = 0.0001) was observed. Pitted red cell counts did not change. Annual rates of vaso-occlusive crisis (p = 0.0105), acute chest syndrome (p = 0.0417), transfusions administered (p = 0.0168), and days in hospital (p = 0.0017) all decreased significantly.Conclusions: Hydroxyurea in children is associated with sustained excellent compliance and monitoring this compliance is uncomplicated. Splenic function in most hydroxyurea-treated children did not change over 1 year of therapy.