The authors studied the long-term clinical and hematological response to hydroxyurea (HU) therapy in young patients, with either S/beta-thalassemia (beta(thal)) (8 patients) or SS (6 patients). All patients with S/beta(thal) responded well to treatment. Longitudinal evaluation of Hb, HbF, and MCV showed a significant increase compared to baseline levels, but the pattern of HbF changes varied among patients. Changes in HbF and Hb correlated well with baseline HbF. Favorable clinical responses, as documented by decline in hospitalization days for vasoocclusive crisis and transfused units of packed red blood cells, were also noted. During treatment, 1 patient was diagnosed with Hodgkin's lymphoma and 2 patients developed bilateral avascular necrosis of the femoral head. HU seems to be effective in a high proportion of young patients with sickle cell disease and in particular with S/beta(thal), but cannot eliminate occurrence of serious adverse events.