The effects of preoperative administration of non-steroidal anti-inflammatory drugs, intraoperative infusion of plasma expanders, type of anaesthesia, blood group and certain coagulation parameters on the bleeding tendency of 354 patients undergoing abdominal and urological surgery were analyzed. The bleeding tendency was estimated by the surgeon at the end of the operation. The preoperative use of non-steroidal anti-inflammatory drugs was associated with an increased bleeding tendency (p less than 0.05). Excessive bleeding was more common in patients who had received dextran intraoperatively, but did not reach statistical significance. Other potential risk factors were not associated with increased bleeding tendency. Two months after the operation, factors VIII:C and vWF:Ag of 32 patients were analyzed. Increased bleeding tendency was not more common in patients with low F VIII activity associated with blood group O, than in others. We conclude that the disorder of primary haemostasis caused by non-steroidal anti-inflammatory drugs is common and causes increased bleeding tendency in surgical patients. If the patient has additional risks of haemostasis, or if accurate surgical haemostasis is of particular importance, it seems justified to withdraw non-steroidal anti-inflammatory drug medication before operation.