We performed a prospective, blinded study to assess and compare the values of preoperative contrast venography and magnetic resonance venography in the detection of deep venous thrombosis in the thigh and pelvis of forty-five consecutive patients who had a displaced acetabular fracture. The magnetic resonance venography and contrast venography were performed an average of seven days (range, one to twenty-nine days) after the injury. Twenty-four asymptomatic thrombi were identified with magnetic resonance venography in fifteen (33 percent) of the patients. Four of the thrombi were in the superficial femoral vein, nine were in the common femoral vein, one was in the external iliac vein, seven were in the internal iliac vein, and three were in the common iliac vein. Ten (42 percent) of the twenty-four thrombi were confirmed with contrast venography; nine of them were located in the thigh. The remaining fourteen thrombi (58 percent) that had been noted on magnetic resonance venography could not be seen with contrast venography because they were located either in the deep pelvic veins or in the uninjured extremity. The thrombi in the internal iliac vein were identified only with magnetic resonance venography. Twelve of the fifteen patients who had thrombi had a filter placed in the inferior vena cava preoperatively. In eight of these patients, the filter was placed because of the findings of magnetic resonance venography alone. Magnetic resonance venography resulted in a change in the therapeutic management of ten (22 per cent) of the forty-five patients. There were no pulmonary emboli. We concluded that magnetic resonance venography is superior to contrast venography for the preoperative evaluation of proximal deep venous thrombosis in patients who have an acetabular fracture. Magnetic resonance venography is non-invasive, does not require the use of contrast medium, images the proximal aspects of both lower extremities simultaneously, and, most importantly, allows for the identification of deep venous thrombosis in the pelvis.