Ultrasound screening for deep vein thrombosis (DVT) after orthopedic surgery revealed thrombosis in up to 30% of patients, but the clinical impact of screening for distal thrombosis has not been established. Three hundred and forty six patients after hip or knee replacement and postoperative standard prophylaxis of 10 days low molecular weight heparin (LMWH) were randomized to receive either prolonged prophylaxis with LMWH or postoperative ultrasound screening for proximal and distal thrombosis. All thrombosis found in screening was treated. Patients with a negative screening test did not receive any heparin after day 10. The cumulative rate of proximal thrombosis and symptomatic pulmonary embolism until day 35 postoperatively was determined. In 174 screening patients, ultrasound found 55 cases of distal (31.6%) and 9 cases of proximal (5.1%) in-hospital thrombosis. In 108 screening patients without thrombosis at discharge 7 new distal (6.5%) and 6 new proximal (5.6%) thrombotic events occurred by day 35. In 163 patients with prolonged prophylaxis, 33 cases of distal (20.2%) and 7 cases of proximal (4,3%) thrombotic events occurred by day 35. The cumulative rate of proximal events was 8,7% in the screening group and 4.3% in the prophylaxis group (p=0.12). In conclusion, ultrasound screening for thrombosis does not reduce the rate of proximal thrombosis and symptomatic pulmonary embolism 35 days after hip or knee replacement surgery.