Anticoagulation treatment is effective in the prevention of stroke as well as deep venous thrombosis (DVT) and pulmonary embolism (PE). Its preventive benefit has to be balanced against possible bleeding complications. We sought to assess risk factors for the severity of bleeding events in patients under anticoagulant treatment. Clinical characteristics, type of anticoagulant treatment, bleeding site, risk factors and additional medication taken were analysed in patients with bleeding complications during an observation period of 12 months. Eighty-seven bleeding complications in 84 patients (mean age, 79 years; 51% female) were observed from January to December 2005 at the Department of Internal Medicine of the Cantonal Hospital of Aargau. Most bleeding complications occurred in the gastrointestinal tract (54%). The median time interval from the beginning of the anticoagulant treatment to the bleeding event was 34 months. Forty-nine percent of events occurred after a treatment time above 36 months. Age was not found to influence the severity of bleeding but the duration of anticoagulant treatment before the occurrence of a complication was significantly longer for older patients (p = 0.001). Our study shows no influence of age on severity of bleeding complications. Furthermore, in patients with advanced age complications occurred later in the treatment course than in younger patients. Overall we assessed various bleeding events in patients treated for over three years. Therefore we emphasize the importance of closely controlling patients on anticoagulant treatment in the later course of treatment and to take account of the anticoagulation when ordering new medication.