Intraoperative, unfractionated heparin, administered intravenously before the femoral work, has demonstrated to be effective in reducing the strong thrombotic stimulus that occurs during total hip arthroplasty (THA) surgery. This randomized, double-blind, prospective study included only THA patients with significant comorbidities predisposing them to deep vein thrombosis (DVT). The 2 groups consisted of study patients who received a single dose of intravenous, intraoperative, unfractionated heparin and control patients who received a single dose of intravenous, intraoperative saline. Magnetic resonance venography was used as the DVT diagnostic tool. The overall prevalence of proximal femoral vein clots was 2.2% (3 of 134), whereas pelvic thrombosis was detected in 10.4% (14 of 134). This study demonstrated that pelvic thrombi may form following THA and that a single dose of intraoperative heparin does not prevent their formation, but may be effective at preventing ipsilateral femoral thrombi. This study strongly supports a multimodal approach to DVT prophylaxis following THA.