Hydroxyurea (HU), a chemotherapeutic agent, used increasingly in the treatment of sickle cell disease (SCD) stimulates the release of a tumor necrosis factor (TNF-alpha) from human macrophages in vitro and the concentration of TNF-alpha is greater than normal in subjects affected by SCD. It is widely accepted that HU may inhibit vaso-occlusive crisis (VOC) by stimulating the production of fetal hemoglobin (HbF) and nitric oxide (NO) in SCD; however, the beneficial effects of HU in vivo may be counteracted by the release of TNF-alpha and, in turn, the expression of a vascular cell adhesion molecule (VCAM-1) on leukocytes. Previous studies have shown that the severity of SCD increases with the leukocyte count. Therefore, we examined the relationship between plasma levels of TNF-alpha and HbF in SCD patients during steady-state (StSt) conditions (in the absence of VOC) and during VOC conditions after the acute administration of HU. Venous blood was collected in SCD patients over 6 h after administering a single dose of HU. Plasma TNF-alpha was found to be greater in SCD subjects than in reported normal adult controls (p<0.05). TNF-alpha in the StSt group was not significantly different than in the VOC group; however, the plasma TNF-alpha tended to greater in the VOC group (p>0.1). An increase in the HbF concentration after acute administration of HU (p<0.01) was not associated with a significant change in plasma TNF-alpha (p>0.1). Contrary to the results of in vitro studies, HU did not increase the plasma concentration of TNF-alpha. These findings suggest that a HU-induced increase in TNF-alpha does not contribute to VOC and sickle cell patients can be counseled that the HU-induced increase in TNF-alpha does not counteract the beneficial effects of HU in SCD.