Background: Epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor is a standard of care for EGFR mutation-positive, untreated metastatic non-small cell lung cancer (NSCLC). However, the efficacy and safety of adjuvant gefitinib for patients with completely resected lung cancer harboring EGFR mutation over cisplatin-based adjuvant chemotherapy were not known in 2011 when this study was initiated. Methods: From September 2011 to December 2015, we randomly assigned 234 patients with completely resected, EGFR mutation-positive (exon 19 deletion or L858R), stage II–III NSCLC to receive either gefitinib (250 mg, once daily) for 24 months or cisplatin (80 mg/m² on day 1) plus vinorelbine (25 mg/m² on days 1 and 8) (cis/vin) every 3 weeks for four cycles. The primary endpoint was disease-free survival (DFS) according to a central review in the intent-to-treat (ITT) population. Results: Two patients in the gefitinib arm withdrew consent and were excluded from the ITT population. No treatment-related deaths were seen in the gefitinib arm, but three treatment-related deaths were reported in the cis/vin arm. Median duration of follow-up was 71 months. Median DFS was numerically longer in the gefitinib arm (36 months) than in the cis/vin arm (25.2 months). However, Kaplan-Meier curves began to overlap around 5 years after surgery, and no significant difference in DFS was seen, with a hazard ratio (HR) of 0.92 (95% confidence interval (CI), 0.67–1.28; P = 0.63). Overall survival was also not significantly different (median not reached in either arm). Five-year survival rates for gefitinib and cis/vin arms were 78.0% and 74.6%, respectively, with an HR for death of 1.03; 95%CI, 0.65–1.65; P = 0.89. Exploratory subset analysis revealed that patients ³70 years old in the gefitinib arm (n = 19/27 with G to cis/vin) survived longer than those in the cis/vin arm (HR 0.31; 95%CI, 0.10–0.98; P = 0.046). Conclusions: Adjuvant gefitinib appeared to prevent early relapse, but did not significantly prolong DFS or OS in patients with completely resected stage II–III, EGFR-mutated NSCLC. The apparent non-inferiority of DFS/OS may justify the use of adjuvant gefitinib in selected subset of patients, especially those deemed unsuitable for cis/vin adjuvant therapy.Clinical trial information: UMIN000006252.