Patients with bleeding disorders frequently need medical or surgical care. The case is reported of a man with von Willebrand's disease type I undergoing radical cystectomy with urethrectomy for multicentric bladder cancer with neoplastic involvement of prostatic urethra, who developed serious bleeding complications which can not be predicted with conventional coagulation in laboratory. The use of the thromboelastograph (TEG) in the critical postoperative period was decisive. The tracing alterations allowed to assess the clotting disorder, constantly counterbalancing the baseline deficit and the blood loss.