Abstract: Objective To investigate the correlation between pulmonary fibrosis and Krebs von den lungen-6(KL-6) level in serum and bronchoalveolar lavage fluid(BALF)in mice exposed to silica.Methods Sixty C57BL/6 mice(half male and half female)were divided randomly into silica-treated group and control group.The mice in silica-treated group were injected with silica suspension and those in the control with normal saline through trachea.The mice were sacrificed at 30th,45th,60th,75th,and 90th day after the silica exposure.KL-6 in the serum and BALF was determined with enzyme-linked immunosorbent assay(ELISA).The lung tissues were fixed and stained with Masson stain for the evaluation of pulmonary fibrosis.Results Collagen deposition in lung tissue was observed as early as 45 days after the silica exposure.Compared with the mice 45 days after the silica-treatement,the scores of pulmonary fibrosis were all significantly increased for the silica-treated mice at 60th,75th,and 90th day after the exposure(P<0.01 for all).For the mice of 60,75 and 90 days after the silica-treatment,the serum KL-6 levels were obviously higher than that of the control and the mice of 45 days after the silica-exposure(P<0.01).For the mice of 75 and 90 days after the silica-treatment,the KL-6 levels in BALF were obviously higher than those of the control and silica-treated mice 60 days after the exposure(P<0.01 for all)and serum and BALF KL-6 levels were positively correlated with the scores of pulmonary fibrosis(r=0.866 or 0.737,P<0.01).Conclusion Pulmonary fibrosis occurs 45 days after silica exposure and serum and BALF KL-6 level is positively correlated with the severity of pulmonary fibrosis in mice.