In sickle cell disease (SCD), the events originating from hemoglobin S polymerization and intravascular sickling lead to reperfusion injury, hemolysis, decreased nitric oxide (NO) bioavailability, and oxidative stress. Oxidative stress is implicated as a contributing factor to multiple organ damage in SCD. We hypothesize that inhibition of sickling by genetic manipulation to enhance antisickling fetal hemoglobin (HbF) expression will have an ameliorating effect on oxidative stress by decreasing intravascular sickling and hemolysis and enhancing NO bioavailability. We tested this hypothesis in BERK (Berkeley) mice expressing exclusively human alpha- and beta(S)-globins and varying levels of HbF, i.e., BERK (<1% HbF), BERKgammaM (20% HbF) and BERKgammaH (40% HbF). Intravascular sickling showed a distinct decrease with increased expression of HbF, which was accompanied by decreased hemolysis and increased NO metabolites (NO(x)) levels. Consistent with decreased intravascular sickling and increased NO bioavailability, BERKgammaM and BERKgammaH mice showed markedly decreased lipid peroxidation accompanied by increased activity/levels of antioxidants [superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx), and reduced glutathione (GSH)] in the muscle, kidney, and liver compared with BERK mice (P < 0.05-0.0001). NO(x) levels showed a strong inverse correlation with hemolytic rate and oxidative stress. Decreased oxidative stress in the presence of elevated HbF levels led to an anti-inflammatory effect as evidenced by decreased peripheral leukocyte counts. These results show that the protective effect of HbF is mediated primarily by decreasing intravascular sickling resulting in decreased oxidative stress and increased NO bioavailability.