Shown are concentrations of fetal hemoglobin in a patient with sickle cell disease treated initially with valproic acid over 5 months and subsequuently with oral hydroxyurea at an initial dose of 11 mg/kg/d (arrows). Fetal hemoglobin at the start of valproic acid therapy was 8%, increasing to 9% during treatment. The patient discontinued valproic acid and began hydroxyurea. During the subsequent 11 months of hydroxyurea therapy, a steady increase in fetal hemoglobin was noted; the present value of fetal hemoglobin, during therapy with 16.2 mg of hydroxyurea per kilogram body weight per day, is 23.7%.