Two hundred twenty-five SCD children have been enrolled in a study assessing the tolerability of hydroxyurea treatment. Mean age at inclusion was 9.2+/-4.4 years, median duration of treatment was 3.8 years. Ten and 75 patients have been treated respectively for more than 10 and 5 years. No severe side effect was related to hydroxyurea treatment, which was discontinued in 81 children mainly for treatment failure (30 cases) or non-compliance (17 cases). Treatment was also withdrawn in 5 of 6 children who had developed hypersplenism, in 3 because of a pathological transcranial Doppler, and in 2 after a stroke.