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Summary Background Bleeding is the most serious complication of the use of oral anticoagulation in the prevention and treatment of thromoboembolic complications. We studied the frequency of bleeding complications in outpatients treated routinely in anticoagulation clinics. Methods In a prospective cohort from thirty-four Italian anticoagulation clinics, 2745 consecutive patients were studied from the start of their oral anticoagulation (warfarin in 64%, acenocourmarol in the rest). The target anticoagulation-intensity was low (international normalised ratio [INR] ≤2·8) in 71% of the patients and high (> 2·8) in the remainder. We recorded demographic details and the main indication for treatment and, every 3·4 months, INR and outcome events. Such events included all complications (bleeding, thrombosis, other), although only bleeding events are reported here, and deaths. We divided bleeding into major and minor categories. Findings 43% of the patients were women. Nearly three-fifths of the patients were aged 60·79; 8% were over 80. The main indication for treatment was venous thrombolism (33%), followed by non-ischaemic heart disease (17%). Mean follow-up was 267 days. Over 2011 patient-years of follow-up, 153 bleeding complications occurred (7·6 per 100 patient-years). 5 were fatal (all cerebral haemorrhages, 0·25 per 100 patient-years), 23 were major (1·1), and 125 were minor (6·2). The rate of events was similar between sexes, coumarin type, size of enrolling centre, and target INR. The rate was higher in older patients: 10·5 per 100 patient-years in those aged 70 or over, 6·0 In those aged under 70 (relative risk 1·75, 95% CI 1·29–2·39, p vs 6·0 per 100 patient-years) (1·80, 12–2·7, p vs 6·3, 1·75, 1·27–2·44, p 4 5, 7·91, 5·44–11·5, p Interpretation We saw fewer bleeding events than those recorded in other observational and experimental studies. Oral anticoagulation has become safer in recent years, especially if monitored in anticoagulation clinics. Caution is required in elderly patients and anticoagulation intensity should be closely monitored to reduce periods of overdosing.