Coagulation and fibrinolysis abnormalities are observed in acute lung injury in both human disease and animal models and may contribute to ongoing inflammation in the lung. Tissue factor (TF), the main initiator of the coagulation cascade, is upregulated in the lungs of patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and likely contributes to fibrin deposition in the airspace. The mechanisms that govern TF upregulation and activation in the lung are not well understood. In the vascular space, TF-bearing microparticles (MPs) are central to clot formation and propagation. We hypothesized that TF-bearing MPs in the lungs of patients with ARDS contribute to the procoagulant phenotype in the airspace during acute injury and that the alveolar epithelium is one potential source of TF MPs. We studied pulmonary edema fluid collected from patients with ARDS compared to a control group of patients with hydrostatic pulmonary edema. Patients with ARDS have higher concentrations of MPs in the lung compared to patients with hydrostatic edema (25.5 IQR 21.3-46.9 vs. 7.8 IQR 2.3-27.5 µmol/l p=0.009 by Mann-Whitney U). These MPs are enriched for TF, have procoagulant activity, and likely originate from the alveolar epithelium (as measured by elevated levels of RAGE (receptor for advanced glycation end products) in ARDS MPs compared to hydrostatic MPs). Furthermore, alveolar epithelial cells in culture release procoagulant TF MPs in response to a proinflammatory stimulus. These findings suggest that alveolar epithelial derived MPs are one potential source of TF procoagulant activity in the airspace in ARDS and that epithelial MP formation and release may represent a unique therapeutic target in ARDS.