Prophylactic efficacy and safety of a low molecular weight heparin (LMWH) and those of conventional unfractionated heparin (UH) were investigated in a randomized study. Totally, 167 consecutive patients undergoing total hip replacement were allocated to two groups. Patients in the LMWH-group (n = 83) received a fixed dose of enoxaparin 40 mg once daily, starting 12 hours preoperatively and continuing for 10 days. Patients in the UH-group (n = 84) received UH 5000 IU twice a day subcutaneously (sc), starting two hours before operation and continuing for 10 days. Deep venous thrombosis (DVT) was diagnosed by bilateral ultrasonography and confirmed by venography. Proximal DVTs were observed in four patients of UH-group (4.8%) and in one of LMWH-group (1.2%, P > 0.05). There was only one pulmonary embolism (PE) in a patient belonging to UH-group (1.2%). Low rates of thromboembolic events could be explained, in addition to heparin prophylaxis, also by early mobilization and regional anaesthesia. Local tolerance (size of haematoma), blood loss and transfusion requirements during the operation and the postoperative period did not show differences between the two study groups. The results of our study indicate that enoxaparin once daily is an effective and safe form of DVT prophylaxis in patients undergoing elective hip replacement.