We have recently shown that insulin-like growth factor-binding protein (IGFBP)-5 is overexpressed in idiopathic pulmonary fibrosis lung tissues and increases collagen and fibronectin deposition. Here, we further examined the effect of IGFBP-5 in vivo by intratracheal administration of replication-deficient adenovirus expressing human IGFBP-5 (Ad5), IGFBP-3 (Ad3), or no cDNA (cAd) to wild-type mice. Increased cellular infiltration and extracellular matrix deposition were observed in mice after Ad5 administration compared with Ad3 and cAd. Mononuclear cell infiltration consisted predominantly of T lymphocytes at day 8. By day 14, the number of infiltrating T cells decreased, whereas that of B cells and monocytes/macrophages increased. IGFBP-5 also induced migration of peripheral blood mononuclear cells in vitro, suggesting that in vivo mononuclear cell infiltration may be the direct result of IGFBP-5 expression. alpha-Smooth muscle actin and Mucin-1 co-localized in cells of mice treated with Ad5, suggesting that IGFBP-5 induced epithelial-mesenchymal transition. In addition, exogenous IGFBP-5 induced alpha-smooth muscle actin expression in primary fibroblasts and epithelial-mesenchymal transition of pulmonary epithelial cells in vitro. In conclusion, our results suggest that overexpression of IGFBP-5 in mouse lung results in fibroblast activation, increased extracellular matrix deposition, and myofibroblastic changes. Thus, the IGFBP-5-induced fibrotic phenotype in vivo may represent a novel model to better understand the pathogenesis of fibrosis.