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Abstract Background In our previous randomised phase 2 study for patients with gemcitabine-refractory advanced pancreatic cancer, S-1 plus leucovorin improved progression-free survival compared with S-1 alone. Here, we evaluated the efficacy of TAS-118 (S-1 plus leucovorin) versus S-1 in overall survival (OS). Patients and methods This randomised, open-label, phase 3 study was conducted at 58 centres in Japan and Korea. Patients with metastatic pancreatic cancer that progressed during first-line gemcitabine-based chemotherapy or recurred during or after post-operative gemcitabine-based adjuvant treatment were randomly assigned (1:1) to receive either S-1 (40–60 mg, twice daily for 4 weeks in a 6-week cycle) or TAS-118 (S-1 40–60 mg plus leucovorin 25 mg, twice daily for 1 week in a 2-week cycle). The primary end-point was OS. Results A total of 603 patients were randomised, and 300 and 301 patients received TAS-118 and S-1, respectively. There was no difference in OS between groups (median OS for TAS-118 versus S-1, 7.6 months versus 7.9 months; hazard ratio [HR], 0.98 [95% confidence interval (CI), 0.82–1.16]; P = 0.756). Progression-free survival was significantly longer with TAS-118 than S-1 (median, 3.9 months versus 2.8 months; HR, 0.80 [95% CI, 0.67–0.95]; P = 0.009). There were interactions between Japan and Korea (P = 0.004) and between unresectable and recurrent disease (P = 0.025) in OS. Incidence, profile and severity of adverse events were similar between groups. Conclusion TAS-118 did not improve OS in patients with gemcitabine-refractory advanced pancreatic cancer compared to S-1. Further studies are needed to find patients who have benefit from adding leucovorin to S-1. Highlights • TAS-118 did not improve overall survival for gemcitabine-refractory advanced pancreatic cancer. • TAS-118 improved progression-free survival compared with S-1 monotherapy. • TAS-118 might be more effective than S-1 in some populations of patients. [ABSTRACT FROM AUTHOR]