Background: Gefitinib yields a longer progression-free survival (PFS) period than platinum–doublet chemotherapy as a first–line treatment for patients with advanced non–small–cell lung cancer (NSCLC) harboring an epidermal growth factor receptor (EGFR) activating mutation, but most patients develop resistance against gefitinib after the initial response. We hypothesized that the insertion of platinum–doublet chemotherapy during the initial response could prevent the emergence of acquired resistance and prolong survival, compared with gefitinib alone. Methods: We performed a phase ΙΙ study of the following first–line treatment for patients with advanced NSCLC with EGFR mutation. Gefitinib (250 mg) was administrated on days 1–56. After a two–week rest, three cycles of cisplatin (80 mg/m²) and docetaxel (60 mg/m²) were administered on days 71, 92, and 113. Gefitinib was re–started on day 134 and was continued until progression. The primary endpoint was the two–year PFS rate. The sample size was estimated at 33, and this treatment was considered worthy for further development if more than 11 of the 33 patients who started treatment had a 2–year PFS. Results: Thirty–three Japanese patients were enrolled. Twenty–five patients could re–start gefitinib, 12 achieved a PFS period of over 2 years, and 9 continued to receive the protocol treatment without experiencing progression. The 1–, 2–, and 3–year estimated PFS rates were 59.4%, 37.5%, and 33.8%, respectively, and the median PFS time was 19.2 months. The 1–, 2–, and 3–year estimated survival rates were 90.0%, 82.9%, and 62.4%, respectively, and the median survival time had not been reached at the time of analysis. Treatment–related deaths and unexpected severe toxicities were not seen. Conclusions: Our results indicated that first–line treatment consisting of gefitinib and inserted cisplatin plus docetaxel is promising for patients with advanced NSCLC with EGFR mutation. A phase ΙΙΙ study of this treatment compared with gefitinib alone is warranted. Clinical trial information: 000001738.