This prospective study was undertaken to determine if B-mode ultrasonic imaging of the greater saphenous vein can provide reliable information about anatomy, sites of valves, and location of major tributaries or duplicate systems. Preoperative ultrasonic imaging was performed on 15 consecutive patients prior to in situ saphenous bypass. The findings of the ultrasonic evaluations were compared with the intraoperative findings. Preoperative assessment of the greater saphenous vein was performed with a high resolution real-time B-mode ultrasonic imager. The procedure was performed with the patient seated with the leg extended. The saphenous vein was imaged starting at the ankle. A waterproof pen was used to mark the vein. The locations of valves and tributary vessels were marked and the diameter of the saphenous vein was recorded. These results were compared with operative findings. In all 15 patients, the saphenous vein was imaged. There was 100 per cent accuracy in the detection and location of valve sites. In one patient, the proximal portion of the vein was thrombosed and this was detected by the B-mode scan. There was exact correlation in 14 of 15 limbs (93%). The one error was due to the examiners' failure to identify a triple venous system; only a double venous system was identified by scan in this patient. The actual venous diameter was consistently within 0.5 mm of that measured by the ultrasound imager. In all patients, the ultrasound image demonstrated the correct anatomic location and size of the vein. In conclusion, B-mode ultrasonic imaging provides an accurate description of the anatomy of the saphenous vein, without the morbidity associated with contrast venography.