BACKGROUND Patients undergoing major abdominal surgery are at risk of both bleeding and thrombotic complications and usually receive heparin thromboprophylaxis. METHODS Risk factors for perioperative bleeding were examined in 3,809 patients in a double-blind, randomized trial investigating heparin thromboprophylaxis. The risk factors were modeled by logistic regression, and a risk score was calculated using the significant factors in the model. RESULTS Bleeding was associated with the following factors in the model, given as adjusted odds ratios (ORa [95% confidence interval]): male sex (ORa 1.68 [1.21 to 2.34] P = 0.003), malignancy (ORa 1.69 [1.21 to 2.34] P = 0.008), gynecological surgery (ORa 1.62 [1.12 to 2.35] P = 0.011), and complex surgery (ORa 2.7 [2.02 to 3.62] P < 0.001). The risk of excessive bleeding for 0, 1, 2, and 3 risk factors was 2%, 6%, 11%, and 21%, respectively. CONCLUSIONS The recognition of patients with these risk factors associated with perioperative bleeding should result in increased vigilance and may lead to modification of surgical and medical therapy.