In a prospective study, antithrombin III (AT III) was performed preoperatively, peroperatively immediately after the surgical procedure and daily during the 8 postoperative days in 57 consecutive patients who underwent major abdominal surgery without prophylactic anticoagulant therapy. On d 8, according to the results of a bilateral radiological phlebography, the patients were divided into 2 groups: Group I: presence of deep venous thrombosis (DVT): n = 28 (49%) and Group 2: absence of deep venous thrombosis: n = 29 (52%). The results of the study showed that the preoperative AT III value did not constitute a marker of the postoperative DVT risk. During the postoperative period, AT III level decreased immediately following the intervention and resumed its preoperative value within 8 d. Nevertheless, this evolution was not different in the 2 groups and was not related to the presence of postoperative DVT.