Abstract Warfarin provides effective prophylaxis against postoperative venous thromboembolic complications in adults undergoing elective hip surgery, but at the risk of increased bleeding complications. Although patients on this drug are routinely monitored by prothrombin time (PT), mild elevations of the partial thromboplastin time (PTT) have been reported with warfarin therapy. In a prospective study of 194 patients undergoing elective hip surgery, the authors assessed the incidence of elevation of the PTT above 50 seconds while the patient was receiving warfarin prophylaxis and the effect of this elevation on bleeding complications. The prophylactic warfarin was begun the night prior to surgery. Thirty-eight patients (19.6%) had a PTT greater than 50 seconds (group 1) and 156 had milder or no elevation of the PTT (group 2). The mean maximum PTT in group 1 was 61.2 (± 12.9), versus 39.9 (± 5.0) seconds in group 2. Major postoperative bleeding complications occurred in 26.3% of group 1 patients, versus 4.5% of group 2 ( P < .01). This subset of patients with an abnormal PTT elevation despite appropriate control of the PT is at a significantly increased risk of major postoperative bleeding. This observation may also prove valuable in reducing bleeding complications from warfarin use in nonsurgical patients.