Aberrations of the tumor suppressor genes FHIT and p53 are frequently associated with a wide range of human cancers, including lung cancer. We studied the combined effects of FHIT and p53 proteins on tumor cell proliferation and apoptosis in human non-small cell lung carcinoma (NSCLC) cells in vitro and on tumor growth in animal models by adenoviral vector-mediated cotransfer of wild-type FHIT and p53 genes. We found that the coexpression of FHIT and p53 synergistically inhibited tumor cell proliferation in NSCLC cells in vitro and suppressed the growth of human tumor xenografts in nude mice. Furthermore, we found that this synergistic inhibition of tumor cell growth corresponded with the FHIT-mediated inactivation of MDM2, which thereby blocked the association of MDM2 with p53, thus stabilizing the p53 protein. Our results therefore reveal a novel molecular mechanism consisting of FHIT-mediated tumor suppression and the interaction of FHIT with other cellular components in the pathways regulating p53 activity. These findings show that combination treatment with synergistic tumor-suppressing gene therapy such as Ad-FHIT and Ad-p53 may be an effective therapeutic strategy for NSCLC and other cancers.