The expression of aquaporin-5, the major water channel expressed in alveolar, tracheal, and upper bronchial epithelium, is significantly down-regulated during acute lung injury. In the present study, the expression of aquaporin-5 in two different mouse models of lung fibrosis was evaluated. Lung fibrosis was induced by intratracheal and by subcutaneous infusion of bleomycin. The expression of aquaporin-5 was investigated by immunohistochemical studies and by polymerase chain reaction. There were many cells with loss of aquaporin-5 immunoreactivity in type I alveolar epithelial cells in the mouse models of lung fibrosis. Immunohistochemistry of lung tissue in aquaporin-5 knockout mice revealed a fibrotic phenotype with increased deposition of extracellular collagen type I in thickened alveolar walls. Semiquantitative analysis of aquaporin-5 mRNA expression showed more abundant content of aquaporin-5 in the lung of the normal mouse compared to the mouse with lung fibrosis. The results of this study showed, for the first time, that chronic lung injury and lung fibrosis is associated with decreased protein and mRNA expression of aquaporin-5 in the lung.