Pulmonary fibrin deposition suggests the involvement of coagulation and fibrinolysis in pulmonary inflammation. The present study was designed to investigate the alterations of coagulation and fibrinolysis in rabbits that received thoracic irradiation. Serial bronchoalveolar lavage (BAL) was performed after the irradiation, and procoagulant activity (PCA) and tissue plasminogen activator (t-PA) were measured in BAL fluids. PCA increased from 2 to 8 weeks after irradiation with increased number of macrophages and increased PCA per macrophage. T-PA also increased with a significant difference at 4 weeks compared to controls. Although irradiation activated both PCA and t-PA, PCA increased prior to t-PA and the elevation lasted longer. It was concluded that activation of the coagulation system promotes pulmonary fibrin deposition and may contribute to the progression of pulmonary injury.