Background: Failure to rapidly identify bleeding in trauma patients leads to substantial morbidity and mortality. We aimed to develop and validate a simple bedside score for identifying bleeding patients requiring escalation of care beyond initial resuscitation. Methods: We included patients with major blunt or penetrating trauma, defined as those with an Injury Severity Score greater than 12 or requiring trauma team activation, at The Ottawa Hospital from September 2014 to September 2017. We used logistic regression for derivation. The primary outcome was a composite of the need for massive transfusion, embolization or surgery for hemostasis. We prespecified clinical, laboratory and imaging predictors using findings from our prior systematic review and survey of Canadian traumatologists. We used an AIC-based stepdown procedure based on the Akaike information criterion and regression coefficients to create a 5-variable score for bedside application. We used bootstrap internal validation to assess optimism-corrected performance. Results: We included 890 patients, of whom 133 required a major intervention. The main model comprised systolic blood pressure, clinical examination findings suggestive of hemorrhage, lactate level, focused assessment with sonography in trauma (FAST) and computed tomographic imaging. The C statistic was 0.95, optimism-corrected to 0.94. A simplified Canadian Bleeding (CAN-BLEED) score was devised. A score cut-off of 2 points yielded sensitivity of 97.7% (95% confidence interval [CI] 93.6 to 99.5) and specificity 73.2% (95% CI 69.9 to 76.3). An alternative version that included mechanism of injury rather than CT had lower discriminative ability (C statistic = 0.89). Conclusion: A simple yet promising bleeding score is proposed to identify highrisk patients in need of major intervention for traumatic bleeding and determine the appropriateness of early transfer to specialized trauma centres. Further research is needed to evaluate the performance of the score in other settings, define interrater reliability and evaluate the potential for reduction of time to intervention.