Prophylactic efficacy and safety of a low molecular weight (LMW) heparin against postoperative thromboembolic complications were investigated in a double-blind, randomized study. Totally, 210 consecutive patients undergoing total hip replacement were allocated to two groups. Patients in the heparin group received 50 IU anti-Xa per kilo body weight of Logiparin™ once daily, and patients in the placebo group received one daily injection of saline. Additional prophylaxis in all the patients was thigh-length compression stockings beginning on the day of the operation. Deep vein thrombosis was diagnosed by bilateral ascending phlebography between Days 8 and 10 after the operation. Twenty patients were excluded from the evaluation. Thirty of 93 patients in the heparin group compared with 45 of 97 patients in the placebo group suffered a thromboembolic complication during the study (P = 0.02). The postoperative blood loss and total number of blood transfusions in the heparin group were higher than in the placebo group. However, the observed differences were of no clinical importance. Adverse effects, including bleeding complications and wound hematomas, were observed in 13 heparin patients and 7 placebo patients (NS). One patient in each group died. Thromboprophylaxis with LMW heparin once daily was safe and more effective than the placebo in patients undergoing total hip replacement