Major abdominal surgery without preoperative adequate oral intake with some cephalosporins may result in vitamin K deficiency and bleeding tendency. The aim of this study is to clarify the effect of preoperative fasting on postoperative coagulation factors. We prospectively examined 16 patients who underwent major abdominal surgery. Patients were divided into a preoperative fast group (Group F, n = 7, mean period of preoperative fasting 8.7 days) and a control group (Group C, n = 9). We did not administer vitamin K and initiated feeding after the seventh postoperative day. In Group C, prothrombin time (PT) and Factors II, VII, IX, and X levels were decreased after the surgery to within normal limits. In Group F, the PT and Factors II, VII, and X levels were decreased after the surgery. Abnormal lower levels of PT and Factors II, VII, and X were seen in 67, 33, 67, and 67 per cent of patients after the surgery, respectively. Factors VII and X levels were higher than in Group C by the third postoperative day. The protein induced by vitamin K absence or antagonist-II levels in Group F were increased at all postoperative points. Clinicians should realize that preoperative fasting for as little as 1 week can induce precoagulopathy, resulting in postoperative coagulopathy after major surgery.