Background: Biliary tract cancer (BTC) is a heterogeneous group of diseases commonly diagnosed at advanced stages. Overall survival (OS) is the gold-standard primary endpoint in randomized controlled trials (RCT). However, the increasing use of subsequent lines of therapies and cross-over designs may confound the treatment effect. We therefore explored the use of progression free survival (PFS) as a surrogate endpoint of OS both at the trial-level and at the patient-level. Methods: For the trial-level correlation, we conducted a systematic review of RCTs in advanced BTC following the PRISMA guidelines. We searched PubMed, the Cochrane Central Register of Controlled Trials and ClinicalTrials.gov from database inception to June 2022 and identified all randomized phase II/III trials testing systemic therapies for advanced BTC. We used a weighted linear regression to measure the correlation of log-transformed hazard ratios (HR) of OS and PFS based on trial size and calculated the surrogate threshold effect (STE). We used the IQWiG framework to define the strength of evidence. For the individual-level analysis, we included patients with advanced BTC treated with first and second line chemotherapy in the real-world RETUD registry. We estimated the correlation via the iterative multiple imputation method. Results: From a total of 1992 studies, we identified 32 RCTs including 70 treatment arms and 5140 patients that fulfilled the inclusion criteria. Twenty-three trials were performed in the first line setting and most were phase II RCTs (N = 23). We found a moderate correlation between OS and PFS (R= 0.79, 95% CI 0.61-0.89). The slope of the regression line was 0.62±0.08, indicating that a 10% risk reduction in PFS will result in a 6.2%±0.8% improvement in OS. The STE was 0.69, suggesting that in a hypothetical trial of 400 patients, a PFS HR of 0.69 will likely result in a significant improvement on OS. Regarding the individual-level correlation, a total of 593 patients with advanced BTC were included. The median age of patients was 68y (IQR 59-74), most were males (54%) and received platinum-based chemotherapy (73.1%). In the first line setting, the median OS was 9.7 months (95% CI 8.7-10.5) and median PFS was 5 months (95% CI 4.5-5.5). We observed a strong correlation between PFS and OS (r= 0.84, 95% CI 0.81-0.86). In the second line setting (N = 259), a similar correlation was observed to the first-line setting (r= 0.76, 95% CI 0.72-0.8). Conclusions: At the trial-level, in this analysis treatment effects on PFS were moderately correlated with OS. A HR < 0.69 in PFS suggested that it would likely lead to a significant OS benefit in a hypothetical trial including 400 patients. At the individual-level, PFS and OS were strongly correlated in a real-world cohort. Future validation in patients treated in the context of randomized trials is warranted.