Post-thrombotic syndrome (PTS) is the most common chronic complication of deep vein thrombosis (DVT), with major consequences for patient quality of life and cost of management. Identifying patients at high risk of developing PTS could be useful for its prevention and may lead to more appropriate therapeutic strategies to reduce its incidence and severity. Prognostic tools for predicting risk are very useful for choosing the optimum treatment and improving patient management and are a preliminary step before developing predictive models useful for determining sensitivity to treatment. At present, although several prognostic markers and models have been proposed, it is still difficult to predict who will develop a PTS or a moderate to severe PTS. The development of PTS is multifactorial and depends largely on the extent and severity of venous obstruction which supports the theory of thrombosis burden (DVT-Burden) as a potential prognostic marker for PTS. It therefore seems important to study the association between thrombosis burden and the occurrence of PTS. The Venous Volumetric Index or VVI (Ouriel 1999) will be used for quantifying DVT-Burden. The VVI was constructed by calculating the volume from the diameter and length of 14 venous segments from the calf veins to the inferior vena cava. The VVI has been validated for its ability to discriminate between symptomatic and asymptomatic DVT and has shown superior performance to other methods for quantifying DVT. This study aim to assess the performance of baseline DVT-burden estimated by the VVI score on ultrasound for predicting the occurrence and the severity of PTS as assessed by the Villalta scale at 6 months.