Chronic obstruction of the iliac veins or inferior vena cava can occur as a result of deep vein thrombosis (DVT), or due to extrinsic compression in non-thrombotic iliac vein lesions (NIVLs). This obstruction can manifest as post-thrombotic syndrome (PTS) after DVT or as chronic venous disease (CVD) in NIVL. Despite sparse evidence, rates of venous stenting for PTS and NIVLs are increasing. A pragmatic, observer-blind, multi-centre, randomised-controlled trial for adults with CVD secondary to either PTS or NIVLs randomised to either best endovenous therapy (including venoplasty and deep venous stenting) or standard therapy (compression +/- anticoagulation). Included participants will have chronic venous disease (CEAP classification 3 - 6) secondary to proximal deep venous disease. The primary outcome is severity of venous disease at 6 months as ascertained by the Venous Clinical Severity Score (VCSS).