Objective : Lung cancer arises after a series of morphological changes, which take several years to progress from normal epithelium to invasive cancer. Multiple molecular changes and growth factor production have been documented in lung cancers, both small cell and non-small cell types. Insulin-like growth factors(IGFs) are important mitogenic and anabolic peptides, both in vivo and in vitro, and are thought to be significant autocrine-paracrine factors involved in normal and malignant cell proliferation. In this study, the degree of expression of IGF-1 on the immunohistochemical staining in human non-small cell lung cancer(NSCLC) cells and small cell lung cancer (SCLC) cells were investigated. Methods : Immunohistochemical staining for IGF-1 was performed in 15 cases of small cell carcinoma, 15 cases of squamous cell carcinoma, 15 cases of adenocarcinoma, and 12 cases of bronchoalveolar carcinoma. Results : The expression of IGF-1 on the immunohistochemical staining significantly increased in NSCLC cells than in SCLC cells. Conclusion : These results suggest the expression of IGF-1 in human lung cancer cells. The immunohistochemical staining of IGF-1 in lung cancer cell lines may assist in the differentiation of NSCLC and SCLC.