Since 1973 a prospective trial of antithrombotic agents for the prophylaxis of deep venous thrombosis and pulmonary embolism was undertaken in 386 high risk patients. Five drugs were employed: 1. A combination of aspirin and dipyridamol (95 patients); 2. Dextran 60 (43 patients); 3. Dihydroergotamine (61 patients); 4. Low dose heparin (63 patients); 5. A combination of dihydroergotamine and low dose heparin (twice daily in 61 patients and three times daily in 63 patients). The patients were investigated by I125 fibrinogen uptake test, phlebography, lung scan and careful clinical evaluation before and after operation. In cases of established thrombosis simultaneous anticoagulation with heparin and coumadin was started. In the first group, 32 (34%) thromboses and three pulmonary emboli were detected of which one patient died. In the dextran group, 24 patients (54%) developed thrombosis and there were no fatal pulmonary emboli. In the low dose heparin group we detected 29 (46%) thromboses and three pulmonary emboli. Only the combination of low dose heparin and dihydroergotamine significantly reduced the incidence of thromboembolic complications-15 (25%) thromboses and no pulmonary emboli. The application of this combination three times a day showed no further improvement but more haemorrhagic complications appeared. Since 1975 no fatal pulmonary emboli have occurred in 720 patients undergoing total hip replacement.