Objective: A central role of macrophages in initiating lung ischemia-reperfusion injury is emerging. Tumor necrosis factor-α a is a proinflammatory cytokine secreted mainly by macrophages under various conditions. We hypothesized that tumor necrosis factor-α from resident lung cells is a key initiating factor in pulmonary ischemia-reperfusion injury. Methods: We used an isolated, buffer-perfused lung system to explore the role of tumor necrosis factor-α production by resident lung cells in pulmonary ischemia-reperfusion injury. Lungs from wild-type mice and tumor necrosis factor-α-deficient mice were subjected to 60 minutes of ischemia followed by 60 minutes of reperfusion. Histologic injury scores and measurements of lung compliance, airway resistance, mean pulmonary artery pressure, vascular reactivity, and wet lung weight index were obtained and compared using repeated-measures analysis of variance. Results: Lungs from tumor necrosis factor-α-deficient mice showed significantly less injury in all physiologic parameters throughout the entire 60 minutes of reperfusion compared with lungs from wild-type mice (P < .001). The most notable effects were observed in pulmonary artery pressure and airway resistance. Vascular reactivity (acute vasoconstrictive episodes per 60 minutes) was also blunted in the lungs from tumor necrosis factor-α-deficient mice compared with the lungs from wild-type mice (5.8 responses/hour vs 1.2 responses). Histologic injury scores and wet lung weight index were significantly reduced in lungs from tumor necrosis factor-α-deficient mice. Conclusions: By using the advantages of a nonblood-perfused system, we have focused our investigation on resident lung cells. Our results demonstrate that resident cell-produced tumor necrosis factor-α is a key initiating factor in acute lung ischemia-reperfusion injury.