Chemical thromboprophylaxis in total hip arthroplasty (THA) may increase surgical site bleeding. The drug dose and timing of such therapy is therefore important. We studied two cohorts of 298 and 301 patients undergoing THA. The first group received their first dose of dalteparin sodium 5000 IU subcutaneously the evening before surgery and the second group a half dose six hours postoperatively, followed by 5000 units daily in both groups. Blood loss was reduced by 146 ml from 1230 ml to 1084 ml (p<0.001) with postoperative prophylaxis alone. The number of patients receiving blood transfusion decreased from 53% to 35% (p=0.001), and the use of transfused packed red blood cells was reduced from 1.25 to 0.83 units per patient (p=0.001). The overall rates of non-vascular complications 6 months after surgery were 12% and 11%, bleeding related events 6.0% and 4.0%, and thromboembolic related events were 2.0% and 2.3% in the preoperative and the postoperative cohorts. Two patients died in the preoperative group and one in the postoperative group due to venous and arterial thromboembolism. This study show that 2500 IU dose of dalteparin started 6 hours after surgery significantly reduced blood loss and transfusions compared to 5000 IU dalteparin injected 12 hours before surgery. Few thromboembolic events occurred, and these were equally distributed.