This study prospectively assessed the efficacy of gefitinib and the survival benefit for non-small cell lung cancer (NSCLC) patients with epidermal growth factor receptor (EGFR) mutations.Patients with either recurrent disease after undergoing surgery or advanced NSCLC disease (IIIB or IV) which demonstrated EGFR mutations were eligible for this study. EGFR mutations in exons 19-21 were examined. The patients with EGFR mutations were enrolled in this study after obtaining their informed consent a second time, and they were thereafter treated with gefitinib.EGFR mutations were detected in 20 of 48 patients with NSCLC, and 19 patients were enrolled onto this study and treated with gefitinib. Seven patients had an exon 19 deletion, 10 had L858R, 1 had both, and 1 had an exon 19 deletion and G796A. The overall response rate was 63.2%, and the disease control rate was 89.5%. In patients with an exon 19 del and L858R, the response rates were 71.4% and 60.0%, respectively. The median progression-free survival time was 7.1 months, and the median survival time was 20.0 months. No life-threatening toxicity was observed. Four of five acquired resistant tumors showed an acquired T790M mutation.EGFR mutations in exons 19 or 21 are considered to be a good predictor of the efficacy of gefitinib.