ACCP ϭ American College of Chest Physicians; APTT ϭ activated partial thromboplastin time; ASA ϭ acetylsalicylic acid; ASPECT ϭ Anticoagulants in the Secondary Prevention of Events in Coronary Thrombosis; INR ϭ international normalized ratio; LMW ϭ low molecular weight; PT ϭ prothrombin time; SPAF ϭ Stroke Prevention in Atrial Fibrillation (CHEST 2001; 119:108S-121S) T he major complication of anticoagulant therapy is bleeding. In this review, the incidence of hemorrhage in patients receiving oral anticoagulants or heparin and the clinical and laboratory risk factors that predispose to bleeding are discussed. The focus is on major bleeding and fatal bleeding. Details of the method used to select relevant articles can be found in the five previous symposia of the American College of Chest Physicians (ACCP). 1-5 Bleeding was classified as major if it was intracranial or retroperitoneal, if it led directly to death, or if it resulted in hospitalization or transfusion. 1,2 Although bleeding is the major side effect of anticoagulant therapy, it should not be considered in isolation of potential benefit, ie, reduction in thromboembolism.