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Background— Vitamin K antagonist (VKA) therapy is increasingly being used for the prevention of venous thromboembolism and stroke in atrial fibrillation. Bleeds are the major concern for VKA prescription, especially in very old patients who carry many risk factors for bleeding. We performed a large multicenter prospective observational study that enrolled very old patients to evaluate the quality of anticoagulation and the incidence of bleedings. Methods and Results— The study included 4093 patients ≥80 years of age who were naïve to VKA for thromboprophylaxis of atrial fibrillation or after venous thromboembolism. Patients' demographic and clinical data were collected, and the quality of anticoagulation and the incidence of bleeding were recorded. The follow-up was 9603 patient-years; median age at the beginning of follow-up was 84 years (range, 80 to 102 years). We recorded 179 major bleedings (rate, 1.87 per 100 patient-years), 26 fatal (rate, 0.27 per 100 patient-years). The rate of bleeding was higher in men compared with women (relative risk, 1.4; 95% confidence interval, 1.12 to 1.72; P =0.002) and among patients ≥85 years of age compared with younger patients (relative risk, 1.3; 95% confidence interval, 1.0 to 1.65; P =0.048). Time in therapeutic range was 62% (interquartile range, 49% to 75%). History of bleeding, active cancer, and history of falls were independently associated with bleeding risk in Cox regression analysis. Conclusion— In this large study on very old patients on VKA carefully monitored by anticoagulation clinics, the rate of bleedings was low, suggesting that age in itself should not be considered a contraindication to treatment. Adequate management of VKA therapy in specifically trained center allows very old and frail patients to benefit from VKA thromboprophylaxis.