Background: Antithrombin III (AT III) is a serine protease inhibitor and the mechanism of its anti-inflammatory action is still not understood. In the present study, we aimed to investigate the anti-inflammatory action of AT III on lung injury in a rat model of sepsis. Methods: Three groups of animals were used in this controlled study: the sham-operated group (sham, n = 3) which only underwent a laparotomy; the control group (control, n = 7) which underwent cecal ligation and perforation (CLP); and the AT III-treated group (AT III, n = 6) which underwent CLP and received intravenous (i.v.) 250 U/kg AT III 30 min before induction of sepsis. Rats were killed 24 h after induction of sepsis by needle aspiration of the right ventricle after a sternotomy, and the lungs and trachea were removed en bloc under ether anesthesia. Results: Pulmonary accumulation of polymorphonuclear leukocytes (PMN) was assessed by measuring lung tissue myeloperoxidase (MPO) activity. Lipid peroxidation in lung tissue was assessed by tissue thiobarbituric acid reactive substance (TBARS) levels. The plasma prostacyclin level was assessed by the plasma 6-keto prostaglandin F1α(6-keto-PGF1α) level, which is a stable derivative of prostacyclin. Histopathological changes in lung tissue were assessed by PMN count in the capillaries and alveolar spaces. The lung tissue TBARS level, MPO activity and PMN count in the control group were significantly higher than in the AT III group (P < 0.05). The change in plasma 6-keto-PGF1α level in the AT III group was insignificant compared with the control group (P = 0.15). Conclusions: AT III prevented pulmonary infiltration of PMN and subsequent injury by the endothelial release of prostacyclin in CLP-induced sepsis.