RATIONALE: Idiopathic Pulmonary fibrosis (IPF) is a devastating disease, which remains refractory to current therapies. OBJECTIVES: To characterize the expression and activity of the membrane-anchored serine protease matriptase in IPF in humans and unravel its potential role in human and experimental pulmonary fibrogenesis. METHODS: Matriptase expression was assessed in tissue specimens from IPF patients versus controls using qRT-PCR, immunohistochemistry and Western blotting, while matriptase activity was monitored by fluorogenic substrate cleavage. Matriptase-induced fibroproliferative responses and the receptor involved were characterized in human primary pulmonary fibroblasts by Western blot, viability and migration assays. In the murine model of bleomycin-induced pulmonary fibrosis, the consequences of matriptase depletion, either by using the pharmacological inhibitor camostat mesilate, or by genetic down regulation using matriptase hypomorphic mice, were characterized by quantification of secreted collagen and immunostainings. MEASUREMENTS AND MAIN RESULTS: Matriptase expression and activity were upregulated in IPF and bleomycin-induced pulmonary fibrosis. In cultured human pulmonary fibroblasts, matriptase expression was significantly induced by TGF-β. Further, matriptase elicited signaling via Protease-Activated Receptor-2 (PAR-2), and promoted fibroblast activation, proliferation and migration. In the experimental bleomycin model, matriptase depletion, by the pharmacological inhibitor camostat mesilate or by genetic down-regulation, diminished lung injury, collagen production and TGF-β expression and signaling. CONCLUSIONS: These results implicate increased matriptase expression and activity in the pathogenesis of pulmonary fibrosis in human IPF and in an experimental mouse model. Overall, targeting matriptase, or treatment by camostat mesilate, which is already in clinical use for other diseases, may represent potential therapies for IPF.