Simple SummaryBiliary tract cancer (BTC) carries a poor prognosis. Most patients with BTC will require ‘systemic’ therapy, treating cancer throughout the body to control the disease and extend life while maintaining or improving the quality of life, but not to cure. There is no standard third-line systemic therapy, and few patients remain fit to receive three lines of treatment. This study assessed 97 patients from three academic centres who received three lines of palliative systemic therapy for BTC. Median survival after starting third-line treatment was 6.4 months, and from first-line treatment it was 26.9 months. The region of the biliary tract in which the cancer originated did not significantly affect prognosis. The 10 patients with an identified molecular change ‘driving’ the cancer, who received third-line treatment targeting that change, survived longer (12.5 months, versus 5.9 months for all other included patients). This study provides a benchmark for future clinical trials within BTC.AbstractPhase 3 trials have established standard first-line (1L) and 2L systemic therapy options for patients with advanced biliary cancer (ABC). However, a standard 3L treatment remains undefined. Clinical practice and outcomes for 3L systemic therapy in patients with ABC were therefore evaluated from three academic centres. Included patients were identified using institutional registries; demographics, staging, treatment history, and clinical outcomes were collected. Kaplan–Meier methods were used to assess progression-free survival (PFS) and overall survival (OS). Ninety-seven patients, treated between 2006 and 2022, were included; 61.9% had intrahepatic cholangiocarcinoma. At the time of analysis, there had been 91 deaths. Median PFS from initiating 3L palliative systemic therapy (mPFS3) was 3.1 months (95%CI 2.0–4.1), while mOS3 was 6.4 months (95%CI 5.5–7.3); mOS1 was 26.9 months (95%CI 23.6–30.2). Among patients with a therapy-targeted molecular aberration (10.3%; n = 10; all received in 3L), mOS3 was significantly improved versus all other included patients (12.5 vs. 5.9 months;p= 0.02). No differences in OS1 were demonstrated between anatomical subtypes. Fourth-line systemic therapy was received by 19.6% of patients (n = 19). This international multicentre analysis documents systemic therapy use in this select patient group, and provides a benchmark of outcomes for future trial design.