Objective: Idiopathic pulmonary fibrosis (IPF) is predominantly a lung fibrotic disease of older adults, and the process underlying aging might significantly influence the development of pulmonary fibrosis. Bleomycin-induced lung injury was investigated in murine models of accelerated senescence (SAMP8) and of normal aging (SAMR1). The levels of Th1/Th2 related cytokine were also measured. Methods: Bleomycin or PBS was injected into the tracheal lumen of 12-month-old SAMP8, 4 and 12-month-old SAMR1 mice. Seven, 14 and 28 days after the injection, the mice were killed and the lungs were harvested for pathological examination, hydroxyproline assay and protein detect. Lung TGF-β1 expression was determined by western blot, and the levels of IL-4 and IFN-γ were detected by ELISA. Results: The aggravated bleomycin induced lung injury was observed in 12-month-old SAMP8 compared with 4 and 12-month-old SAMR1. Twenty eight days after injection of bleomycin, Ashcroft score was significant higher in 12-month-old SAMP8 than in 4 and 12-month-old SAMR1 ( P Conclusions: Bleomycin-induced pulmonary fibrosis in SAM was aggravated by aging. The old SAM with bleomycin-induced pulmonary fibrosis might be inclined to Th2-biased immune responses. (This work was funded by National Natural Science Foundation of China Grants 81070046).