Introduction: We performed a prospective, randomized, double-blind study on 40 patients scheduled for primary total hip arthroplasty due to arthrosis or osteonecrosis to determine the effect of tranexamic acid on per- and postoperative blood losses and on the number of blood transfusions needed. Patients and methods: 40 patients were randomized to tranexamic acid (10 mg/kg given as a bolus intravenous injection, followed by a continuous infusion of 1 mg/kg/hour for 10 hours) or placebo (20 mL saline given intravenously) 15 minutes before the incision. We recorded the peroperative and postoperative blood losses at removal of the drain 24 hours after the operation and the number of blood transfusions. Results: Patients receiving tranexamic acid had a mean peroperative blood loss of 480 mL versus 622 mL in patients receiving placebo (p= 0.3), a postoperative blood loss of 334 mL versus 609 mL (p= 0.001), a total blood loss of 814 mL versus 1231 mL (p= 0.001) and a total need for 4 blood transfusions versus 25 (p= 0.04). No patient in either group had symptoms of deep venous thrombosis, pulmonary embolism or prolonged wound drainage. Interpretation: Transemic acid is effective in reducing the postoperative blood loss, the total blood loss and the need for blood transfusion in primary total hip arthroplasty.