We have studied the cellular and molecular responses to long-term hydroxyurea (HU) treatment in 29 severely affected young patients with sickle cell disease (mean age, 10.9 +/- 4.1 years). Patients received HU at 20 mg/kg/d on 4 consecutive days per week initially, with a monthly escalated dose avoiding marrow-toxicity (mean steady-state dose, 34.2 +/- 4.6 mg/kg/d) for 12 to 36 months (mean duration, 22 months). The studied parameters were hemoglobin F (HbF), F reticulocytes (F retics), F cells, the amount of HbF per F cell (F/F cell), polymer tendency at 40% and 70% oxygen saturation, and hemolysis. Initial HbF (Fi) was dispersed (from 0.85% to 13.9%). HbF increased in all patients but 1. HbF at maximal response (Fmax) reached a sustained level varying from a 1.5-fold to a 16-fold Fi after a variable delay (6 to 24 months). Fmax was not related to HU dosage, but triangle upF (Fmax - Fi) was strongly correlated to triangle upMCV (MCVmax - MCVi). HbF increase resulted from the increase of both F cells and F/F cell. In this rather short series, Fi and Fmax were not significantly associated with age, gender, or beta-globin haplotype. Neither Fmax nor triangle upF was related to bone marrow reserve, as measured by baseline reticulocyte or neutrophil counts. However, Fmax was highly dependent on Fi. When patients are individualized into three groups according to Fmax (group 1, Fmax >20% [12 patients]; group 2, 10% < Fmax < 20% [11 patients]; group 3, Fmax <10% [5 patients]), Fi is significantly different between groups, being the highest in group 1. In addition, the best responders (group 1) were significantly different from patients in the two other groups with higher levels of total hemoglobin, decreased bilirubin, and decreased polymer tendency.