The anatomy of the saphenopopliteal junction shows considerable variation, and clinical localisation of this junction is inaccurate. Duplex scanning in preoperative mapping of the saphenous vein system in bypass surgery has been shown to be highly effective. In 62 patients with clinical evidence of insufficiency of the saphenopopliteal junction, preoperative localisation with duplex scanning was performed in 66 extremities. In 62 extremities duplex localisation matched the operative findings and in four extremities a difference of 2 cm or more was found. There was 1 false negative surgical exploration. In 93% of the cases exact localisation of the junction enabled us to perform flush ligation of a small saphenous vein through minimal exposure. Preoperative duplex scanning of the saphenopopliteal junction is highly accurate.