Hydroxyurea (HU) enhances the synthesis of fetal hemoglobin (HbF) and can improve the clinical course of some adult patients with sickle cell anemia (SCA). In a randomized trial, we studied the biologic effects and the clinical benefit of HU in children and young adults with severe SCA. Twenty-five patients (median age, 9 years) were randomized to receive either HU (at the initial dosage of 20 mg/kg/d) or a placebo for 6 months and were then switched to the other arm for the next 6 months. Among the 22 evaluable patients (median age, 8 years), significant increases in HbF and mean corpuscular volume occurred during the HU treatment period. The white blood cell and reticulocytes counts decreased significantly, but these changes were not clinically relevant. Sixteen of 22 patients (73%) experienced a complete disappearance of events requiring hospitalization. The number of days of hospitalization as well as the number of hospitalizations for patients on HU, as compared with those for the patients receiving placebo, were significantly reduced. We conclude that treatment with HU in children and young adults is feasible, well-tolerated, and improves the clinical course of SCA. The long-term effects of HU require further investigation.