Gefitinib is effective in treating advanced non-small cell lung cancer (NSCLC), especially in Asian patients in whom the prevalence of epidermal growth factor receptor (EGFR) mutation was high. We analyzed our gefitinib treatment use in patients for advanced NSCLC to study the influence of clinical factors on the treatment outcomes in a tertiary referral medical center in Taiwan. Clinical data and EGFR mutational status of the tumors were collected. A total of 907 patients received gefitinib for advanced NSCLC: 466 patients (51.4%) underwent testing for EGFR mutations, and the other 441 patients did not. In the 466 patients who were tested for EGFR mutations, 272 (58.4%) had EGFR mutations, and an EGFR mutation was a prominent factor for objective response to gefitinib (67.3% vs. 18.3% in wildtype EGFR, p < 0.001). In the 441 patients who did not receive EGFR mutation sequencing, nonsmoker status, female sex, and adenocarcinoma cell type were predictors for better gefitinib response (p < 0.005). We found that testing for EGFR mutations was helpful in NSCLC patients in Taiwan to guide the use of gefitinib. In patients with positive activating EGFR mutations, gefitinib efficacy was prominent and significant. Therefore, analysis for EGFR mutation should be advocated. In those patients who have unknown EGFR mutation status, demographic and histopathology characteristics can be relied on to judge the potential efficacy of gefitinib use.