The integrin family plays a major role in complex biological events such as differentiation, development, wound healing, and the altered adhesive and invasive properties of tumor cells. Integrin (alpha5beta1 is a classical fibronectin receptor, and it has been known as a tumor suppressor gene because tumor cells overexpressing alpha5beta1 are less tumorigenic than their parent cells. However, this finding conflicts with some recent data that suggests that the emergence of alpha5beta1 expression correlates with the tumor progression. We, therefore, investigated the expression of alpha5beta1 integrin in 20 lung cancer cell lines by flow cytometric analysis and in 88 node-negative non-small cell lung cancers (NSCLCs) by RT-PCR and immunohistochemical assays to determine the significance of this prognostic factor. In the 20 lung cancer cell lines, 8 (40.0%) cell lines strongly expressed integrin alpha5, 3 (15.0%) cell lines had moderate or weak alpha5 expression, and the remaining 9 (45.0%) cell lines expressed no integrin alpha5. In the 88 node-negative NSCLC patients, 44 samples (50.0%) were evaluated as having integrin alpha5 overexpression, and the integrin alpha5 expression was significantly associated with the status of differentiation and the age of the patients (P = 0.0379 and 0.0312, respectively). In the node-negative patients, the overall survival rate for patients with integrin alpha5 overexpressed tumors was significantly worse than for those individuals whose tumors had normal integrin alpha5 expression (P = 0.016).