Outpatients with an episode of SVT of the grand saphenous vein (for at least 4 cm), and/or SVT of the small saphenous vein (for at least 4 cm), and/or SVT of a collateral vein of the large saphenous vein of the thigh (for at least 4cm) are included in this prospective, randomised, double blind, national multicentre study.