BACKGROUND & AIMS: Increasing evidence suggests sexual dimorphism in treatment effects across cancers; however, its impact in biliary tract cancer (BTC) remains unclear. We compared toxicity and efficacy of palliative and adjuvant chemotherapy between male and female patients with BTC. METHODS: We conducted a retrospective cohort analysis of individual patient data from four randomized controlled trials in BTC and English population-based data. Study outcomes were adverse events and overall survival (OS), compared by sex. RESULTS: Among 994 trial participants (49% male, 51% female) included in time-to-event analyses, 770 were evaluable for adverse events. Population data included 3,953 patients (46% male, 54% female) for OS analysis. Females experienced higher rates of grade 3/4 fatigue (odds ratio [OR] 2.18; 95% CI 1.02-4.67; p = 0.045). Higher rates of grade 3/4 vomiting (OR 1.97; 95% CI 1.00-3.91; p = 0.052), nausea (OR 1.99; 95% CI 0.80-4.97; p = 0.14), and fatigue in BILCAP (OR 2.31; 95% CI 0.77-6.88; p = 0.13) were observed in females but were not statistically significant. OS was similar between sexes in ABC trials (hazard ratio [HR] 0.94; 95% CI 0.79-1.11; p = 0.45) and in population data (HR 1.03; 95% CI 0.79-1.11; p = 0.45). In BILCAP, the HR for adjuvant capecitabine vs. observation was 0.71 in males (95% CI 0.50-1.00; p = 0.048) and 0.91 in females (95% CI 0.63-1.32; p = 0.625). Females with gallbladder cancer demonstrated improved OS compared with males in BILCAP (HR 0.48; 95% CI 0.24-0.98; p = 0.04). CONCLUSION: Sex differences in toxicity were observed, with higher rates of grade 3/4 fatigue in females. Survival outcomes were broadly similar; however, females with gallbladder cancer receiving adjuvant capecitabine showed improved survival compared with males. Although population analyses were limited by sample size, these findings warrant consideration in the design and interpretation of future BTC trials. IMPACT AND IMPLICATIONS: This study investigated the impact of biological sex on treatment outcomes in patients receiving chemotherapy for biliary tract cancers, which are typically associated with poor outcomes. Analysis of ABC and BILCAP clinical trials found a higher incidence of severe grade adverse events in women receiving cisplatin/gemcitabine and adjuvant capecitabine, relative to males, whilst overall survival was superior in women in the BILCAP trial. These findings are important for clinicians treating patients with biliary tract cancers and should be considered in the design and analysis of future clinical trials in biliary tract cancer, as the role of biological sex may an important determinant of chemotherapy response.