A large number of studies have demonstrated that the presence of eosinophils in the lungs of patients with pulmonary fibrosis correlates with poor prognosis or resistance to therapy. However, direct evidence of the relationship between the influx of eosinophil and pulmonary fibrosis has not yet been described experimentally. In this article, pulmonary fibrosis was induced by different doses of bleomycin (BLM) and using different aged rats. On selected days afterwards, the lungs were lavaged and harvested for analyzing fibrosis, eosinophil influx and cytokine expression. There was a significant relationship between eosinophilia and the pulmonary fibrosis (r=0.98, p<0.01). In spite of the fact that there was no significant increase in hydroxyproline of the lung, eosinophil influxes of bronchoalveolar lavage fluid (BALF) was maximal 7 d after BLM administration. Moreover, there were similar patterns among transforming growth factor beta (TGF)-beta(1), hepatocyte growth factor (HGF) and eosinophil influx of BALF in that they were dependent on dose of BLM and age. These findings, taken together, have suggested the causal correlation of eosinophilia during the early stage with subsequent pulmonary fibrosis. The possible role of eosinophils in the pathogenesis of pulmonary fibrosis might contribute to not only TGF-beta(1) but also HGF production.