This investigation was undertaken to identify and correlate one factor that makes patients undergoing total hip replacement more susceptible to venous thrombosis and pulmonary embolism than those who have almost any other elective orthopaedic procedure, and to determine why the operation of total hip replacement has proved to be relatively resistant to antithrombotic prophylaxis compared with general surgical procedures. Using the depletion of antithrombin III as a marker of activation of the coagulation system, two groups of patients were compared: twenty-one who were subjected to hip arthroplasty and fourteen who underwent general surgical procedures. Both during and after operation the decrease in the quantity of antithrombin III in hip-arthroplasty patients was significantly greater (p less than 0.05) than the decrease in general surgical patients. Seventy-three per cent of hip-replacement patients had venographic evidence of recent thrombosis, 60 per cent of which were discontinuous femoral-vein thrombi. Femoral-vein thrombosis occurs frequently in hip-arthroplasty patients and is relatively resistant to current antithrombotic prophylaxis. The data presented suggest that during hip surgery there is a strong systemic activation of the clotting cascade that is associated with local vessel injury and local stasis in the femoral vein, an association not found in most general surgical procedures.