Recent reports that activating mutations of the EGFR have a significant association with the response to gefitinib drew much attention. Mutations are more frequently observed in Oriental patients, females, non-smokers and adenocarcinoma patients, which correspond to patient profiles predictive of a good clinical response with gefitinib. In vitro experiments also revealed EGFR mutant cell lines are highly sensitive to gefitinib. It seems that development of tailor-made therapy of lung cancer would be possible by the test for EGFR gene mutations. Furthermore, EGFR mutations are the first molecular change known to specifically occur in lung cancer, preferentially in never smokers, especially in adenocarcinoma that is increasing in incidence. It is ultimately necessary to identify non-tobacco-related carcinogens that cause EGFR mutations for effective prevention of lung cancer.