Surgery in anticoagulated patients is problematic. Coumarin therapy is often discontinued or reversed to reduce the perioperative bleeding risk. Meanwhile, the thromboembolic risk is enhanced. We sought to determine the frequency of bleeding and thromboembolism in anticoagulated patients undergoing routine surgery and to investigate the role of patient characteristics and the level of anticoagulation. We studied patients who attended the Leiden Anticoagulation Clinic for treatment relating to mechanical heart valve prostheses, atrial fibrillation or myocardial infarction and underwent surgery at the Leiden University Medical Centre between 1994 and 1998. Outcome events were bleeding and thromboembolism in the perioperative period. Seventy-two complications occurred in 603 interventions, yielding an overall frequency of 11.9% [95% confidence interval (CI): 9.3-14.9], 9.5% (n = 57) for haemorrhage and 2.5% (n = 15) for thromboembolism. Younger patients tended to have more complications [odds ratio (OR) for >65 years of age: 0.5, 95% CI 0.3-1.0] as did patients with atrial fibrillation (OR for atrial fibrillation versus mechanical heart valve prostheses: 1.8, 95% CI 0.8-4.2). High postoperative levels of anticoagulation were associated with a slightly increased risk of complications [OR international normalized ratio (INR) > 3 vs. INR < 2: 1.3, 95% CI 0.6-3.0]. We conclude that routine surgery in anticoagulated patients yields a high perioperative bleeding and thromboembolic risk. While neither patient characteristics nor the level of anticoagulation appeared to play a major role in the occurrence of complications, the risk was clearly associated to the type of surgery, with the highest risk in thoracic surgery.