Hyperoxia-induced acute lung injury (HALI) is characterized by an influx of inflammatory cells, increased pulmonary permeability, endothelial and epithelial cell death. In a murine model and in vitro, we found Angiopoietin 2 to be a critical mediator of lung oxidant injury, inflammation, edema, and regulator of necrotic cell death pathways. The clinical significance of our findings was highlighted by the detection of increased Angiopoietin 2 levels in patients with ALI.