BACKGROUND In the literature there is a range from 1% to 20 % of duplication (up to 20%) of the great saphenous vein (GSV) reported, because there is a lack of an accurate definition of the GSV and objective parameters for an anatomical identification. OBJECTIVE To investigate the frequency of true duplications of the GSV. MATERIALS AND METHODS A systematic review of the literature, a retrospective analysis of duplex examinations, and a prospective study of duplex examinations to investigate the frequency of true duplications of the GSV. RESULTS In the literature review, a great variety of definitions is used for duplication of the GSV. Before the consensus of the Union International de Phle ' bologie (UIP) in 2006, Only in a small number of studies, the definition of the GSV in the saphenous compartment between the fascial blades is mentioned. CONCLUSION Phlebographic studies have been the criterion standard for the identification of venous anatomy. Now, duplex is regarded as the criterion standard for accurate detection of the veins. True duplication of the GSV is less common than the previous literature has suggested, namely 1.6% to 2%. It is recommended that the duplicated GSV should be treated to avoid an important risk of recurrence of venous insufficiency.