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Background: The Canadian Syncope Risk Score (CSRS) was developed to predict 30-day serious outcomes not evident during emergency department (ED) evaluation., Objective: To externally validate the CSRS and compare it with another validated score, the Osservatorio Epidemiologico della Sincope nel Lazio (OESIL) score., Design: Prospective cohort study., Setting: Large, international, multicenter study recruiting patients in EDs in 8 countries on 3 continents., Participants: Patients with syncope aged 40 years or older presenting to the ED within 12 hours of syncope., Measurements: Composite outcome of serious clinical plus procedural events (primary outcome) and the primary composite outcome excluding procedural interventions (secondary outcome)., Results: Among 2283 patients with a mean age of 68 years, the primary composite outcome occurred in 7.2%, and the composite outcome excluding procedural interventions occurred in 3.1% at 30 days. Prognostic performance of the CSRS was good for both 30-day composite outcomes and better compared with the OESIL score (area under the receiver-operating characteristic curve [AUC], 0.85 [95% CI, 0.83 to 0.88] vs. 0.74 [CI, 0.71 to 0.78] and 0.80 [CI, 0.75 to 0.84] vs. 0.69 [CI, 0.64 to 0.75], respectively). Safety of triage, as measured by the frequency of the primary composite outcome in the low-risk group, was higher using the CSRS (19 of 1388 [0.6%]) versus the OESIL score (17 of 1104 [1.5%]). A simplified model including only the clinician classification of syncope (cardiac syncope, vasovagal syncope, or other) variable at ED discharge-a component of the CSRS-achieved similar discrimination as the CSRS (AUC, 0.83 [CI, 0.80 to 0.87] for the primary composite outcome)., Limitation: Unable to disentangle the influence of other CSRS components on clinician classification of syncope at ED discharge., Conclusion: This international external validation of the CSRS showed good performance in identifying patients at low risk for serious outcomes outside of Canada and superior performance compared with the OESIL score. However, clinician classification of syncope at ED discharge seems to explain much of the performance of the CSRS in this study. The clinical utility of the CSRS remains uncertain., Primary Funding Source: Swiss National Science Foundation & Swiss Heart Foundation.