BACKGROUND: The aim of the study was to assess the regression rate of persistent echogenic masses in patients with idiopathic deep venous thrombosis (DVT) who were after initial 6 months randomized to long-term anticoagulation treatment or to discontinuation of anticoagulation. PATIENTS AND METHODS: We followed 168 patients with idiopathic DVT for two years since diagnosis. After 6 months of standard therapy (heparin/LMWH, warfarin with target INR 2-3) we randomized patients with persistent echogenic masses of over 20% of venous diameter to either discontinuation of warfarin or to continuation of warfarin for another 6 months. We evaluated the size of thrombotic masses with duplex ultrasound. RESULTS: After 6 months of standard therapy complete regression was observed in 69 patients while in 99 patients echogenic masses persisted, with 71 patients maintaining an obstruction of at least 20% of venous cross-sectional area. 52 patients were randomized. 27 patients continued warfarin therapy and in 25 patients warfarin was discontinued. After discontinuation of warfarin a further trend to regression was seen in both groups. A significant difference in regression of thrombotic masses between the 6th and 12th month of follow up was seen in patients continuing to receive warfarin, both within the popliteal vein (2.81 +/- 1.56 vs. 2.10 +/- 1.67 mm, p < 0.05) and the femoral vein (3.95 +/- 2.74 vs. 2.30 +/- 1.34 mm, p < 0.05). CONCLUSIONS: After 6 months of DVT treatment persistent echogenic masses can be found in almost two thirds of patients. Even after discontinuation of anticoagulation treatment echogenic masses further regress. However, the rate of regression in patients with deep venous thrombosis between the 6th and 12th month was greater in patients randomized to continue warfarin during this period than in patients without anticoagulation.