BACKGROUNDThe Adjuvant S-1 for Cholangiocarcinoma Trial (JCOG1202, [ASCOT]) was a multicenter, randomized controlled trial aimed at investigating the efficacy and safety of adjuvant chemotherapy (AC) with S-1 for resected biliary tract cancer (BTC). This trial reported that overall survival was prolonged with AC compared with observation.METHODSWith the aim of increasing enrollment, the present survey biannually recorded the number of patients eligible for enrollment into ASCOT and reasons for ineligibility among patients who had undergone surgery for BTC from April 2015 to September 2017 at 36 institutions participating in ASCOT.RESULTSOf 2039 patients who underwent surgery for BTC, 211 (10.3%) were already enrolled, 166 (8.1%) were eligible but had not been enrolled, and 1662 (81.5%) were ineligible. Among ineligible patients, the predominant reasons for ineligibility were patient refusal (n = 332, 20.0%), pathologic stage (pT1N0; n = 248, 14.9%), age (≥ 81 years; n = 196, 11.8%), and prolonged postoperative complications (n = 176, 10.6%).CONCLUSIONSPatients undergoing surgery for BTC are a heterogeneous cohort comprising patients with earlier pathologic stage, advanced age, and prolonged postoperative complications. These factors should be considered during the design of future clinical trials of perioperative treatments for resectable BTC.