The ultrasonic velocity detector is a noninvasive technique which can be used at the bedside to evaluate the deep venous system of patients with suspected thrombophlebitis. The technique has been used in 112 patients who presented with symptoms and signs suggestive of thrombophlebitis. The results of the ultrasonic testing program have been verified by 67 venograms in 53 patients and by operation in an additional 4 patients. The results of the velocity detector evaluation correlated well with those of venography in 93% of the patients studied. The technique is most accurate and useful in the case of occlusions in the iliofemoral and femoropopliteal venous segments. False negative examinations are most apt to occur with venous occlusions confined to small veins of the calf, which cannot be evaluated with the method.