The dysfunction of alveolar barriers is a critical factor in the development of lung injury and subsequent fibrosis, but the underlying molecular mechanisms remain poorly understood. To clarify the pathogenic roles of tight junctions in lung injury and fibrosis, we examined the altered expression of claudins, the major components of tight junctions, in the lungs of disease models with pulmonary fibrosis. Among the 24 known claudins, claudin-1, claudin-3, claudin-4, claudin-7, and claudin-10 were identified as components of airway tight junctions. Claudin-5 and claudin-18 were identified as components of alveolar tight junctions and were expressed in endothelial and alveolar epithelial cells, respectively. In experimental bleomycin-induced lung injury, the levels of mRNA encoding tight junction proteins were reduced, particularly those of claudin-18. The integrity of the epithelial tight junctions was disturbed in the fibrotic lesions 14 days after the intraperitoneal instillation of bleomycin. These results suggest that bleomycin mainly injured alveolar epithelial cells and impaired alveolar barrier function. In addition, we analyzed the influence of transforming growth factor-β (TGF-β), a critical mediator of pulmonary fibrosis that is upregulated after bleomycin-induced lung injury, on tight junctions in vitro. The addition of TGF-β decreased the expression of claudin-5 in human umbilical vein endothelial cells and disrupted the tight junctions of epithelial cells (A549). These results suggest that bleomycin-induced lung injury causes pathogenic alterations in tight junctions and that such alterations seem to be induced by TGF-β.