OBJECTIVES: The aim of this study was to develop a practical risk score to predict the risk and implications of major bleeding in acute coronary syndromes (ACS). BACKGROUND: Hemorrhagic complications have been strongly linked with subsequent mortality in patients with ACS. METHODS: A total of 17,421 patients with ACS (including non-ST-segment elevation myocardial infarction [MI], ST-segment elevation MI, and biomarker negative ACS) were studied in the ACUITY (Acute Catheterization and Urgent Intervention Triage strategY) and the HORIZONS-AMI (Harmonizing Outcomes with RevasculariZatiON and Stents in Acute Myocardial Infarction) trials. An integer risk score for major bleeding within 30 days was developed from a multivariable logistic regression model. RESULTS: Non-coronary artery bypass graft surgery (CABG)-related major bleeding within 30 days occurred in 744 patients (7.3%) and had 6 independent baseline predictors (female sex, advanced age, elevated serum creatinine and white blood cell count, anemia, non-ST-segment elevation MI, or ST-segment elevation MI) and 1 treatment-related variable (use of heparin + a glycoprotein IIb/IIIa inhibitor rather than bivalirudin alone) (model c-statistic = 0.74). The integer risk score differentiated patients with a 30-day rate of non-CABG-related major bleeding ranging from 1% to over 40%. In a time-updated covariate-adjusted Cox proportional hazards regression model, major bleeding was an independent predictor of a 3.2-fold increase in mortality. The link to mortality risk was strongest for non-CABG-related Thrombolysis In Myocardial Infarction (TIMI)-defined major bleeding followed by non-TIMI major bleeding with or without blood transfusions, whereas isolated large hematomas and CABG-related bleeding were not significantly associated with subsequent mortality. CONCLUSIONS: Patients with ACS have marked variation in their risk of major bleeding. A simple risk score based on 6 baseline measures plus anticoagulation regimen identifies patients at increased risk for non-CABG-related bleeding and subsequent 1-year mortality, for whom appropriate treatment strategies can be implemented.