Gefitinib (Iressa, ZD 1839) is an orally bioavailable small molecule that selectively inhibits epidermal growth factor receptor(EGFR) tyrosine kinase activity. Gefitinib causes a dramatic response in approximately 10% to 20% of patients with non-small-cell lung cancer (NSCLC) who receive prior chemotherapy. Studies of gefitinib in combination chemotherapy in first-line therapy of advanced NSCLC have, however, failed to show improvement of survival. Gefitinib also failed to prolong survival in a placebo controlled clinical trial for patients with pretreated advanced NSCLC. In addition, gefitinib did not improve survival as maintenance therapy after chemoradiation in patients with Stage III NSCLC. A possible explanation for the lack of a survival benefit seen in these studies might be failure to select of patients suitable for gefitinib treatment. Empirically, and also in phase II trials, a good clinical response has been observed most frequently in women, nonsmokers, patients with adenocarcinomas, and East Asian patients. Recently, mutations and amplifications of the EGFR gene identified in a subset of NSCLC have been reported to be useful for prediction of enhanced sensitivity to gefitinib. It is also known that some recurrent tumors have a secondary mutation in the EGFR kinase domain, T 790 M, conferring drug resistance. In Japan, a significant number of patients often develop fatal interstitial lung disease after the introduction of gefitinib, although it is, in general, well tolerated. In the future, we must demonstrated benefits of gefitinib treatment in prospective clinical trials by recruiting patients selected on the basis of biological characteristics. It is also important to further elucidate various issues that include other determinants of gefitinib sensitivity, other mechanisms of resistance to gefitinib or mechanisms or predictive factors of interstitial lung disease by close collaboration among clinicians and basic researchers.