Hyperoxia-induced lung injury complicates the care of many critically ill patients who receive supplemental oxygen therapy. Hyperoxic injury to lung tissues is mediated by reactive oxygen species, inflammatory cell activation, and release of cytotoxic cytokines. IFN- is known to be induced in lungs exposed to high concentrations of oxygen; however, its contribution to hyperoxia-induced lung injury remains unclear. To determine whether IFN- contributes to hyperoxia-induced lung injury, we first used anti-mouse IFN- antibody to blockade IFN- activity. Administration of anti-mouse IFN- antibody inhibited hyperoxia-induced increases in pulmonary alveolar permeability and neutrophil migration into lung air spaces. To confirm that IFN- contributes to hyperoxic lung injury, we then simultaneously exposed IFN- deficient (IFN- -/-) mice and wild type mice to hyperoxia. In the early phase of hyperoxia, permeability changes and neutrophil migration were significantly reduced in IFN- -/- mice compared with wild-type mice, although the differences in permeability changes and neutrophil migration between IFN- -/- mice and wild type mice were not significant in the late phase of hyperoxia. The concentrations of IL-12 and IL-18, two cytokines which play a role in IFN- induction, significantly increased in bronchoalveolar lavage fluid after exposure to hyperoxia in both IFN- -/- mice and wild type mice, suggesting that hyperoxia initiates upstream events that result in IFN- production. Although there was no significant difference in overall survival, IFN- -/- mice had a better early survival rate than did the wild-type mice. Therefore, these data strongly suggest that IFN- is a key molecular contributor to hyperoxia-induced lung injury.