Aims: Clinical trials have shown that anticoagulation with vitamin K antagonists (VKAs), e.g. warfarin, decreases the risk of stroke in patients with atrial fibrillation (AF); however, increased bleeding risk is one of the safety concerns. The primary objective was to conduct a systematic review of the published literature, assessing the risk of major bleeding and mortality in patients with AF treated with VKAs. Methods and results: Online searches of MEDLINE, EMBASE, BIOSIS, and the Cochrane Library were performed to a pre-specified protocol from 1960 to March 2012 for randomized controlled trials (RCTs) and from January 1990 to March 2012 for observational studies. A total of 47 studies (16 RCTs and 31 observational studies) were included. Cumulative follow-up was 61,563 patient-years for RCTs and 484 241 patient-years for observational studies. The overall median incidence of major bleeding was 2.1 per 100 patient-years (range, 0.9-3.4 per 100 patient-years) for RCTs and 2.0 per 100 patient-years (range, 0.2-7.6 per 100 patient-years) for observational studies. With study year as a proxy for changing management patterns, some evidence of bleeding rates and/or their reporting increasing over time was noted. Mortality rates from observational studies were inadequately reported to allow comparison with those from RCT data. Conclusion: The median rate of major bleeding in observational studies and RCTs is similar. The larger heterogeneity in bleeding rates observed in a real-life setting could reflect a high variability in standard of care of patients on VKAs and/or methodological differences between observational studies and/or variability in data sources.