Background: The efficacy of S-1 monotherapy as adjuvant chemotherapy (AC) for pancreatic cancer has been demonstrated, and the long-term results are clearly improving.¹ In biliary tract cancer, the JCOG1201 trial in Japan also demonstrated the usefulness of S-1 AC recently.² We have been also performed S-1 AC for biliary tract cancer in recent years in accordance with the clinical necessity. In this article, we herein retrospectively analyze the S-1 AC for biliary tract cancer especially in the significance of initiation day or therapy continuation. Methods: 134 patients with the resection of biliary tract cancer (intrahepatic cholangiocarcinoma 25/ hilar 17/ gallbladder 27/ distal bile duct 38/ papillary 25 from January 2017 to December 2021 were included in this study. Age 73(30-87) years, 47 hepatic resections/66 pancreatecticoduodenectomies/21 peri-gallbladder resection. The decision to administer AC was made at the discretion of the attending doctor and in consultation with the patient, and the decision to discontinue was based on the patient's physical condition. If there was no particular need to discontinue, the treatment was continued for six months at least. Results: S-1 AC was performed in 57/137 (43%) of eligible patients. Age was similar between with or without AC patients (73 vs. 73 years old). There were no significant differences by the cancer location. Although pathological stage was higher in the patients with AC (stage 0:1:2:3:4 = 0: 20: 39: 31: 9%) than without AC ( = 9: 45: 33: 10: 3%) (p< 0.001), 2 years recurrence free survival (2y- RFS) was similar between with AC or without AC patients (66.6 vs 71.6%, p=0.82). In the subgroup analysis of the patients with AC, although comparisons of induction day of AC within or after postoperative 6 weeks showed no difference in patient’s prognosis, patients with continuation of AC for at least 6 months after initiation clearly had a better prognosis than without it (2y-RFS: 89.6 vs 41.7%, p=0.003). Conclusions: In S-1 AC for the patients after biliary tract cancer resection, early induction does not play much role, but continuation is important to maintain better prognosis. 1. Uesaka et al. Lancet, 2016. 2. Nakachi et al. Lancet, 2023.