Alveolar epithelial cells are known to be present at the primary site of lung damage in pulmonary fibrosis. Apoptosis has been implicated as being involved in epithelial cell damage and pulmonary fibrosis. Because the cyclin-dependent kinase inhibitor p21 induces G1 arrest and DNA repair and because it also prevents apoptosis in some cells, we hypothesized that p21 gene transfer may attenuate bleomycin-induced pulmonary fibrosis in mice, the pathogenesis of which likely involves epithelial cell apoptosis. Human p21 protein was expressed in mouse alveolar epithelial cells at 1-7 days in vitro and was detected predominantly in lung epithelial cells at 1-7 days in vivo after adenoviral transfer of the human p21 gene. Inflammatory cell infiltration and fibrosis had already begun at 7 days in this model. Adenoviral transfer of the human p21 gene at 7 days after intratracheal instillation of bleomycin led to a decrease in the number of apoptotic cells, lung inflammation, and fibrosis at 14 days. Therefore, the forced expression of p21 exerted both anti-apoptotic and anti-fibrotic effects, which would facilitate the ultimate goal of treatment for pulmonary fibrosis.