Thrombi found in the deep thigh veins of postoperative total hip or knee arthroplasty patients were followed prospectively by repeat venous ultrasonography to determine the efficacy and appropriate duration of anticoagulation therapy. Forty-four patients who had 47 proximal vein thrombi (femoral or popliteal) were treated with heparin or warfarin or both. Thirteen patients had two or more and 34 had a single follow-up venous ultrasound scan. The last follow-up scan was done at an average of seven weeks after the thrombus was diagnosed. Thirty-four thrombi (72%) had lysed at that time. Twelve thrombi (26%) were smaller or unchanged in size. One thrombus propagated and later embolized despite the use of heparin and warfarin. Forty-five percent (21 of 47) of the thrombi had lysed within six weeks of the initiation of anti-coagulation therapy. The results of this study indicate that the standard duration of anticoagulation therapy for postoperative proximal deep vein thrombosis of three months may be excessively long for half of these patients. The use of follow-up venous ultrasound scanning to determine when it is appropriate to terminate anticoagulation therapy is a logical clinical management strategy.