The technique of impedance phlebography for the detection of deep vein thrombi was evaluated. A total of 410 examinations were performed on 111 patients. Approximately 20% of these patients were not suitable candidates for this technique. Twenty-one patients underwent radiographic venography by a method that routinely visualizes all of the named deep veins. Of these 21 patients having venograms, eight had false negative impedance studies. The frequent occurrence of false negative impedance studies casts doubt on the value of impedance phlebography for the diagnosis of suspected deep vein thrombosis or for the screening of high-risk patient groups.