165 patients undergoing unilateral primary THA, who had comorbidity that predisposed them for deep vein thrombosis (DVT), were randomized to receive a single intraoperative injection of heparin or saline. Over a minimum of 3 months, the 2 groups did not differ in the occurrence of pelvic thrombi. There was also no difference between the two groups in the mean size of thrombi, intraoperative blood loss, and post-operative swelling. However, none of the heparin group patients developed an ipsilateral femoral thrombus.The study, therefore, concluded that heparin may prevent ipsilateral femoral thrombi, but not pelvic thrombi.