Tumor cells that have acquired resistance to gefitinib may complicate the future treatment of patients with non-small cell lung cancer (NSCLC). To investigate the mechanisms of acquired resistance, an acquired gefitinib-resistant cell line, PC-9/ZD2001, has been established using a gefitinib-sensitive NSCLC cell line, PC-9. PC-9/ZD2001 showed collateral sensitivity to tumor necrosis factor (TNF)-alpha. Bortezomib is a proteasome inhibitor and enhances TNF-alpha-induced cell death. These observations suggest that the combination of bortezomib and TNF-alpha might have effects against gefitinib-resistant cells. To verify this hypothesis, a combination effect between these drugs was examined using MTT assay and immunoblotting. This combination showed synergistic cytotoxic effect in NSCLC cell lines with either acquired or intrinsic gefitinib resistance. However, this combination effect was not observed in gefitinib-sensitive cells. On the other hand, bortezomib inhibited TNF-alpha-induced IkappaB degradation in all cell lines. From these observations, it is concluded that the combination of bortezomib and TNF-alpha could be used to overcome gefitinib-resistance.