Background: Gemcitabine plus cisplatin (GC) therapy has been the standard palliative chemotherapy for patients with advanced biliary tract cancer (BTC). Our previous KHBO1002 trial, a single-arm phase II study of GC plus S-1 combination therapy (GCS), demonstrated a favorable survival benefit in BTC patients (Kanai M, et al. Cancer Chemother. Pharmacol. (2015) 75, 293-300). This phase III study aims to confirm the superiority of GCS to GC in terms of overall survival in patients with recurrent or unresectable BTC. Methods: Eligibility criteria include chemotherapy-naïve patients with recurrent or unresectable biliary tract adenocarcinoma (gallbladder, intrahepatic biliary tract, extrahepatic biliary tract, or ampulla of Vater), an Eastern Cooperative Oncology Group performance status of 0–2, and adequate organ function. Eligible patients are randomized into either GCS arm or GC arm. In the GCS arm, gemcitabine and cisplatin were administered intravenously at doses of 1,000 or 25 mg/m (2), respectively, on day 1, and oral S-1 was administered daily at a dose of 80 mg/m (2) on days 1-7 every 2 weeks. In the GC arm, 1000 mg/m2 of gemcitabine and 25 mg/m2 of cisplatin are infused on days 1 and 8 and repeated every 3 weeks. The primary endpoint is overall survival and the secondary endpoints are progression-free survival, %response rate, %planned dose, adverse events. The sample size was calculated to be 220 (110 patients per arm), assumed median survival time of 11.2 months in GC and of 16.2 months in GCS, an accrual period of 4 years, and a follow-up period of 1 year. Forty-nine institutions are participating in this study (NCT02182778☐AUMIN 000014371). The study was activated in August 2014. Clinical trial information: NCT02182778