In the present study, we demonstrated the following: (1) co-inoculation of mice with lung cancer cells and lung fibroblasts results in higher susceptibility to tumorigenesis and higher tumor progression in those mice, (2) lung fibroblast-conditioned medium contributes to lung cancer cell survival through, at least in part, the production of HGF, (3) inhibition of HGF/Met signaling can suppress cancer cell survival and tumor progression, and (4) lung cancer cells stimulate HGF production in lung fibroblasts, as summarized in Fig. 7. Here, MET is linked to cancer.