Fibroblast growth factor (FGF)-9 belongs to the FGF family which modulate cell proliferation, differentiation, and motility. Benzo(a)pyrene is a polycyclic aromatic hydrocarbon (PAH) and ubiquitous environmental carcinogen present in automobile exhaust, cigarette smoke, and foods. The major purposes of this study were to explore the roles of FGF-9 in the benzo(a)pyrene-induced lung cancer invasion in vitro and the metastatic development of lung adenocarcinoma in human. The data of RT-PCR analysis indicated that treatments of human lung adenocarcinoma CL5 cells with benzo(a)pyrene and a PAH mixture motorcycle exhaust particulate (MEP) extracts increased FGF-9 mRNA expression. The increased expression was blocked by cotreatments with a p38 mitogen-activated protein kinase inhibitor SB202190 and an extracellular signal-regulated kinase inhibitor PD98059. The results of immunoblot analysis and Matrigel assay showed that benzo(a)pyrene and MEP extracts produced a concomitant induction of FGF-9 protein and invasive ability of CL5 cells. The benzo(a)pyrene- and MEP-induced invasion was suppressed by FGF-9 neutralizing antibodies. The results of immunohistochemistry analysis of human lung adenocarcinoma specimens showed that FGF-9 protein was detected in the adenocarcinoma cells but not in normal epithelium. FGF-9 staining intensity was positively correlated with status of disease and degree of lymph node metastasis in these lung adenocarcinomas. These present findings suggest that FGF-9 has potential roles in benzo(a)pyrene-induced CL5 cell invasion and human lung adenocarcinoma metastasis.