Background: The prognosis of patients with advanced biliary tract cancer who have progressed on gemcitabine plus cisplatin is dismal. We aimed to investigate the efficacy and safety of second-line liposomal irinotecan plus fluorouracil and leucovorin in patients with metastatic biliary tract cancer that has progressed on gemcitabine plus cisplatin., Methods: This multicentre, open-label, randomised, phase 2b (NIFTY) study was done at five academic institutions in South Korea and included patients aged 19 years or older with histologically or cytologically confirmed metastatic biliary tract cancer that had progressed on first-line gemcitabine plus cisplatin and an Eastern Cooperative Oncology Group performance status of 0 or 1. By use of an interactive web-based response system integrated with an electronic data capture system, patients were randomly assigned (1:1) using permuted blocks (block size 4) to receive either intravenous liposomal irinotecan (70 mg/m 2 for 90 min) plus intravenous leucovorin (400 mg/m 2 for 30 min) and intravenous fluorouracil (2400 mg/m 2 for 46 h) every 2 weeks or leucovorin and fluorouracil only every 2 weeks, and were stratified by primary tumour site, previous surgery with curative intent, and participating centre. Study treatment was continued until the patient had disease progression or unacceptable toxicities, or withdrew consent. The primary endpoint was blinded independent central review (BICR)-assessed progression-free survival. The primary endpoint and safety were assessed in the full analysis set and the safety analysis set, respectively, both of which comprised all randomly assigned patients who received at least one dose of the study treatment. This trial is registered with ClinicalTrials.gov, NCT03524508, and enrolment is complete., Findings: Between Sept 5, 2018, and Feb 18, 2020, 193 patients were screened for eligibility, of whom 174 (88 in the liposomal irinotecan plus fluorouracil and leucovorin group and 86 in the fluorouracil plus leucovorin group) were enrolled and included in the full analysis and safety analysis sets. At a median follow-up of 11·8 months (IQR 7·7-18·7), the median BICR-assessed progression-free survival was significantly longer in the liposomal irinotecan plus fluorouracil and leucovorin group (7·1 months, 95% CI 3·6-8·8) than in the fluorouracil and leucovorin group (1·4 months, 1·2-1·5; hazard ratio 0·56, 95% CI 0·39-0·81; p=0·0019). The most common grade 3-4 adverse events were neutropenia (21 [24%] of 88 in the liposomal irinotecan plus fluorouracil and leucovorin group vs one [1%] of 86 in the fluorouracil and leucovorin group) and fatigue or asthenia (11 [13%] vs three [3%]). Serious adverse events occurred in 37 (42%) patients receiving liposomal irinotecan plus fluorouracil and leucovorin and 21 (24%) patients receiving fluorouracil and leucovorin. There were no treatment-related deaths., Interpretation: Adding liposomal irinotecan to fluorouracil and leucovorin significantly improved BICR-assessed progression-free survival in patients with advanced biliary tract cancer. Liposomal irinotecan plus fluorouracil and leucovorin could be considered a standard-of-care second-line therapy for advanced biliary tract cancer., Funding: Servier and HK inno., N Translation: For the Korean translation of the abstract see Supplementary Materials section., Competing Interests: Declaration of interests CY received honoraria from Servier, Bayer, AstraZeneca, Merck Sharp & Dohme, Eisai, Celgene, Bristol Myers Squibb, Debiopharm, Ipsen, Kyowa Kirin, Novartis, Boryung Pharmaceuticals, Merck Serono, Mundipharma, Roche, and Janssen, and received research grants from Servier, Bayer, AstraZeneca, Ono Pharmaceuticals, Celgene, Ipsen, Boryung Pharmaceuticals, Ildong Pharmaceuticals, CKD Pharmaceuticals, and HK inno.N. JHJ received honoraria from Boryung Pharmaceutical, Daewoong Pharmaceutical, Eisai, HK inno.N, Lilly, Novartis, Pfizer, and Roche. JC received honoraria from Roche, Bayer, Eisai, Ipsen, and Bristol Myers Squibb, and received research grants from Bayer and Dong-A Pharmaceuticals. GKAA reports research grants from Arcus, Agios, AstraZeneca, BioNtech, Bristol Myers Squibb, Celgene, Flatiron, Genentech/Roche, Genoscience, Incyte, Polaris, Puma, QED, Silenseed, and Yiviva, and personal consultation fees from Adicet, AstraZeneca, Alnylam, Autem, Bayer, Beigene, Berry Genomics, Cend, Celgene, CytomX, Eisai, Eli Lilly, Exelixis, Flatiron, Genentech/Roche, Genoscience, Helio, Incyte, Ipsen, Legend Biotech, Merck, Nerviano, QED, Redhill, Rafael, Servier, Silenseed, Sillajen, Sobi, Surface Oncology, Therabionics, Vector, and Yiviva. All other authors declare no competing interests., (Copyright © 2021 Elsevier Ltd. All rights reserved.)