Bilateral venography was performed between 12 and 15 days after total hip replacement in 745 consecutive patients, all of whom had heparin prophylaxis. Of these, 81 patients (10.8%) showed evidence of recent deep vein thrombosis: 23 (3%) distal, 44 (5.9%) isolated proximal, five (0.7%) both proximal and distal, and nine (1.2%) extensive thrombosis from calf to thigh. Compared with previous reports heparin appeared to have reduced the number of distal and contralateral thromboses, but was far less effective in reducing proximal femoral thrombosis. In a cadaver study, the femoral veins were inspected during simulated total hip replacement by either an anterior or a posterior approach. In every case the femoral vein became kinked or folded in the thigh position imposed during the preparation of the femur. Local damage appears to be an important factor in proximal thrombosis; care at operation could help to minimise trauma to the femoral veins and reduce the number of such cases.