Effects of hydroxyurea therapy on resting energy expenditure (REE) in children with sickle cell disease have not been evaluated. Eight children with sickle cell disease were examined before hydroxyurea therapy and again 6.9 +/- 3.5 months after hydroxyurea initiation. Resting energy expenditure, dietary intake, and growth were assessed. In six children, baseline REE was elevated, and REE decreased an average of 17% with hydroxyurea. This was associated with a significant increase in fetal hemoglobin. These pilot data suggest that hydroxyurea may curtail the hypermetabolic state observed in children with sickle cell disease and may offer a clinically important secondary benefit.