Several studies have suggested that epidermal growth factor receptor (EGFR) gene mutation, EGFR gene amplification, and some other biomarkers may be predictors of gefitinib sensitivity. We analyzed EGFR mutation and EGFR copy number in 22 gefitinib-treated non-small cell lung cancer (NSCLC) cases and their relation to the survival of patients. We also studied 143 gefitinib-naïve Japanese NSCLC cases. The erbB2 copy number was also studied in 59 gefitinib-naïve NSCLC cases. In gefitinib-treated patients, the presence of EGFR mutation was associated with a higher response rate to gefitinib and a longer overall survival, but the increased EGFR gene copy number was not. In gefitinib-naïve cases, EGFR mutation but not EGFR gene copy number was significantly correlated with gender, pathological subtypes, and smoking status. The erbB2 copy number was not significantly correlated with the EGFR mutation or EGFR copy number in 59 cases. In conclusion, EGFR mutation was a better predictor of clinical outcome in gefitinib-treated patients than the EGFR gene copy number.