Gefitinib (Iressa, ZD1839) is a selective epidermal growth factor receptor tyrosine kinase inhibitor that can block growth factor-mediated cell proliferation and extracellular signal-regulated kinases 1/2 (ERK1/2) activation. High-level Rad51 expression has been reported in chemoresistant or radioresistant carcinomas. In this study, we examined the role of Rad51 in regulating the response to gefitinib among different human lung cancer cell lines. The H520 line (human squamous cell carcinoma) was less sensitive to gefitinib compared with the H1650 (human adenocarcinoma) or A549 (human bronchioloalveolar carcinoma) lines. In H1650 and A549 cells but not in H520 cells, gefitinib decreased cellular levels of phospho-ERK1/2 and Rad51 protein and message levels. Moreover, gefitinib decreased Rad51 protein levels by enhancing Rad51 protein instability through 26S proteasome-mediated degradation. Inhibition of endogenous Rad51 levels by si-Rad51 RNA transfection significantly enhanced gefitinib-induced cytotoxicity. In contrast, transfection with constitutively active MKK1 vector could restore both Rad51 protein levels and cell survival inhibited by gefitinib. The MKK1/2-ERK1/2 signaling pathway constitutes the upstream signaling for maintaining Rad51 message and protein levels. Rad51 protein can protect lung cancer cells from cytotoxic effects induced by gefitinib. Suppression of Rad51 may be a novel lung cancer therapeutic modality to overcome drug resistance to gefitinib.