Low-molecular-weight heparins (LMWHs) have been shown to be effective in the prevention of deep vein thrombosis (DVT) after major orthopaedic surgery, such as total hip replacement (THR). The efficacy and safety of two LMWHs, reviparin and enoxaparin, were compared in a prospective, double-blind, double-dummy study involving 498 patients undergoing total hip replacement. Drugs were given preoperatively in doses of 4200IU anti-Xa for reviparin and 40 mg (approximately 4000IU anti-Xa) for enoxaparin. The endpoint for the assessment of efficacy was venographically confirmed DVT. The endpoint for the assessment of safety was clinically important bleeding during study treatment. There were evaluable venograms for 460 patients (93%). Of these 460 patients only 416 fulfilled the study protocol. A total of 39 DVTs (9%) occurred in this per protocol group of patients, 21 (10%) in the reviparin group, and 18 (9%) in the enoxaparin group. The incidence of proximal DVT was 6% in each group. The two treatments were found to be equivalent in terms of efficacy. For the 460 patients with venograms (intent-to-treat) venous thrombosis occurred in 49 patients (11%). Of the 230 patients randomly assigned to reviparin, 27 had a DVT (12%), whereas 22 of the 230 enoxaparin patients (10%) had a DVT. The incidence of proximal DVT was 6% in both groups. Again, the two treatment groups were clinically equivalent in efficacy. Major bleeding complications occurred in two enoxaparin- and one reviparin-treated patient. Peri- and postoperative blood loss and blood transfusions were similar in both treatment groups. The reviparin-treated patients had fewer haematomas, bruisings and higher red cell counts and lower haemoglobin levels than the enoxaparin-treated patients.