Every clinician managing a patient with venous thromboembolism of the lower limbs is faced with two opposing problems: first the risk of antivitamin K induced haemorrhage requires adequate but not excessive hypocoaguability of limited duration (international normalized ratio between 2 and 3); second the threat of recurrence requiring an adequate level of hypocoaguability of sufficient duration. Recently reported clinical data have greatly changed management attitudes. Current recommendations favour a 6 week regimen of antivitamin K for distal venous thrombosis in patients with thromboembolism of lower limb veins without any other aggravating factor and a 12 week regimen for proximal vein thrombosis or pulmonary emboli, although the therapeutic efficacity and risk remain to be demonstrated with precision. In France, we are conducting a multicentric controlled study with sufficient power (1800 patients) comparing parallel groups of patients: those with distal deep venous thrombosis treated 6 versus 12 week regimens, those with proximal deep venous thrombosis and/or pulmonary emboli treated 12 versus 24 week regimens. The "DOTAVK" study (Durée Optimale du Traitement Antivitamine K) involves patients with first time venous thrombosis or pulmonary emboli and no underlying neoplasia or coagulation disease. The two criteria of outcome are haemorrhage complications and thromboembolic recurrence during treatment and the first year after treatment withdrawal.