Background: Accurate 1-year bleeding risk estimation after hospital discharge for acute coronary syndrome(ACS) may help clinicians guide the type and duration of antithrombotic therapy. Currently there are no predictive models for this purpose. The aim of this study was to derive and validate a simple clinical tool for bedside risk estimation of 1-year post-discharge serious bleeding in ACS patients. Methods: The risk score was derived and internally validated in the BleeMACS (Bleeding complications in a Multicenter registry of patients discharged with diagnosis of Acute Coronary Syndrome) registry, an observational international registry involving 15,401 patients surviving admission for ACS and undergoing percutaneous coronary intervention (PCI) from 2003 to 2014, engaging 15 hospitals from 10 countries located in America, Europe and Asia. External validation was conducted in the SWEDEHEART population, with 96,239 ACS patients underwent PCI and 93,150 without PCI. Results: Seven independent predictors of bleeding were identified and included in the BleeMACS score: age, hypertension, vascular disease, history of bleeding, malignancy, creatinine and hemoglobin. The BleeMACS risk score exhibited a C-statistic value of 0.71 (95% CI 0.68-0.74) in the derivation cohort and 0.72 (95% CI 0.67-0.76) in the internal validation sample. In the SWEDEHEART external validation cohort, the C-statistic was 0.65 (95% CI 0.64-0.66) for PCI patients and 0.63 (95% CI 0.62-0.64) for non-PCI patients. The calibration was excellent in the derivation and validation cohorts. Conclusions: The BleeMACS bleeding risk score is a simple tool useful for identifying those ACS patients at higher risk of serious 1-year post-discharge bleeding. (c) 2017 Elsevier B.V. All rights reserved.