This study aimed to analyze whether Ulinastatin, a urinary trypsin inhibitor (UTI), inhibits the TGF-beta signaling pathway and lung fibrosis induced by thoracic irradiation in a lung injury mouse model. The thoraces of 9-week-old female fibrosis-sensitive C57BL/6 mice were irradiated with a single X-ray dose of 12 Gy or 24 Gy. UTI was administrated intraperitoneally at a dose of 200,000 units/kg concurrently with radiation (concurrent UTI) or daily during the post-irradiation period for 8-14 days (post-RT UTI). Mice were sacrificed at 16 weeks after irradiation to assess the histological grade of lung fibrosis and immunohistochemical TGF-beta expression. Survival rates of mice given 24 Gy to the whole lung +/- UTI were also compared. Post-RT UTI reduced the score of lung fibrosis in mice, but concurrent UTI had no beneficial effects in irradiated mice. The fibrosis score in post-RT UTI mice was 3.2 +/- 1.0, which was significantly smaller than that of irradiated mice without UTI treatment (RT alone; 6.0 +/- 1.3; p < 0.01). The rates of TGF-beta positive cells in post-RT UTI and the RT alone mice were 0.18 +/- 0.03 and 0.23 +/- 0.04, respectively (p < 0.01). There was a significantly positive correlation between the fibrosis score and the TGF-beta positive rate (R(2) = 0.26, p < 0.01). The survival rate at 30 weeks for post-RT UTI mice was significantly better than that of RT alone mice (33% vs. 10%, p < 0.05). The administration of post-RT UTI suppressed TGF-beta expression and radiation-induced lung fibrosis, which resulted in significant survival prolongation of the irradiated mice.