Deep vein thrombosis (DVT) is a severe disease, and conventional treatment with low molecular weight heparin (LMWH) and warfarin is associated with some degree of long-term sequelae, i.e. post-thrombotic syndrome (PTS). Catheter-directed thrombolytic (CDT) therapy has been introduced worldwide the last two decades. Reports have suggested a beneficial effect of this costly treatment, but there are no randomized clinical trials documenting its short- and long-term efficacy and safety. This multi-center study will randomize patients with acute iliofemoral vein thrombosis to either conventional treatment or CDT in addition to conventional treatment. Main outcome parameters are patency rates at 6 months and prevalence of PTS at 24 months. The main short-term hypothesis is that CDT of first-time acute DVT will increase patency of the affected segments after 6 months from <50% to >80%. The main long-term hypothesis is that CDT will improve long-term functional outcome, i.e. risk of PTS after 2 years from >25% to <10%.