Acute respiratory distress syndrome (ARDS) is a serious complication of sepsis. Thrombomodulin, an important endothelial anticoagulant, binds thrombin to generate activated protein C (APC). We have previously demonstrated that APC prevents endotoxin (ET)-induced pulmonary vascular injury by inhibiting activated leukocytes. We therefore examined whether recombinant human soluble thrombomodulin (rhs-TM) prevents activated leukocyte-induced pulmonary vascular injury in rats receiving ET. Intravenous administration of rhs-TM prevented ET-induced pulmonary accumulation of leukocytes and increase in pulmonary vascular permeability, as well as ET-induced histological changes, such as leukocyte infiltration and pulmonary interstitial edema. Dansyl-Glu-Gly-Arg-chloromethyl ketone-treated factor Xa (DEGR-Xa), a selective inhibitor of thrombin generation, did not prevent these effects of ET. rhs-TM did not prevent ET-induced pulmonary accumulation of leukocytes and pulmonary vascular injury in rats pretreated with DEGR-Xa. These results suggest that rhs-TM prevents ET-induced pulmonary vascular injury by inhibiting pulmonary accumulation of leukocytes and that this effect may be mediated primarily by APC generation.