Background: Cisplatin is preferred to carboplatin when treating metastatic urothelial carcinoma of the bladder (mUCB), despite its greater toxicity. Randomised studies underpinning this have been performed in noncontemporary populations with limitations in sample sizes and analyses, affecting their validity in current clinical practice. Objective: To estimate overall survival (OS) and assess the benefit of cisplatin-based regimens over carboplatin-based regimens in a contemporary cohort of patients with mUCB. Design, setting, and participants: A nationwide retrospective cohort study was conducted in patients diagnosed with de novo mUCB in the Netherlands between 2016 and 2019, who underwent first-line treatment with cisplatin- or carboplatin-based chemotherapy, based on the data from the Netherlands Cancer Registry. Outcome measurements and statistical analysis: A propensity model for receiving cisplatin-based chemotherapy based on age, sex, age-adjusted Charlson Comorbidity Index, renal function, performance status, serum haemoglobin, and the presence of visceral and bone metastases was used to produce inverse probability weighting (IPW) per patient. Unadjusted and IPW-adjusted Kaplan-Meier OS curves of both chemotherapy groups were compared by restricted mean survival time (RMST). Results and limitations: Of the 1041 patients with mUCB, 359 received either cisplatin (n = 170; 47%) or carboplatin (n = 189; 53%) as first line. The cisplatin group was younger, had fewer comorbidities, and had better performance status and renal function. The median OS in the cisplatin and carboplatin groups was 13.1 and 11.5 mo, respectively. After IPW adjustment, prognostic factors were balanced between the two chemotherapy groups (standardised differences <0.1), and differences in RMST were <2.0 mo and not statistically significant up to 24 mo. Conclusions: After accounting for all known prognostic factors, we found no significant survival benefit for cisplatin over carboplatin as first-line chemotherapy in mUCB. Patient ...