Acute lung injury (ALI) is characterized by increased alveolar cytokines, inflammatory cell infiltration, oxidative stress, and alveolar cell apoptosis. Previous work suggested that xanthine oxidoreductase (XOR) may contribute to oxidative stress in ALI as a product of the vascular endothelial cell. We present evidence that cytokine induced lung inflammation and injury involves activation of XOR in the newly recruited mononuclear phagocytes (MNP). We found that XOR was increased predominantly in the MNP that increase rapidly in the lungs of rats that develop ALI following intratracheal cytokine insufflation. XOR was recovered from the MNP largely converted to its oxygen radical generating, reversible O-form, and alveolar MNP exhibited increased oxidative stress as evidenced by increased nitrotyrosine staining. Cytokine insufflation also increased alveolar cell apoptosis. A functional role for XOR in cytokine-induced inflammation was demonstrated when feeding rats two different XOR inhibitors, tungsten and allopurinol, decreased MNP XOR induction, nitrotyrosine staining, inflammatory cell infiltration, and alveolar cell apoptosis. Transfer of control or allopurinol treated MNP into rat lungs confirmed a specific role for MNP XOR in promoting lung inflammation. These data indicate that XOR can contribute to lung inflammation by its expression and conversion in a highly mobile inflammatory cell population.