BACKGROUND: Because only limited evidence suggests that elastic stockings prevent the post-thrombotic syndrome in patients with symptomatic deep venous thrombosis (DVT), these stockings are not widely used. OBJECTIVE: To evaluate the efficacy of compression elastic stockings for prevention of the post-thrombotic syndrome in patients with proximal DVT. DESIGN: Randomized, controlled clinical trial. SETTING: University hospital. PATIENTS: 180 consecutive patients with a first episode of symptomatic proximal DVT who received conventional anticoagulant treatment. INTERVENTIONS: Before discharge, patients were randomly assigned to wear or not wear below-knee compression elastic stockings (30 to 40 mm Hg at the ankle) for 2 years. Follow-up was performed for up to 5 years. MEASUREMENTS: The presence and severity of the post-thrombotic syndrome were scored by using a standardized scale. RESULTS: Post-thrombotic sequelae developed in 44 of 90 controls (severe in 10) and in 23 of 90 patients wearing elastic stockings (severe in 3). All but 1 event developed in the first 2 years. The cumulative incidence of the post-thrombotic syndrome in the control group versus the elastic stockings group was 40.0% (95% CI, 29.9% to 50.1%) versus 21.1% (CI, 12.7% to 29.5%) after 6 months, 46.7% (CI, 36.4% to 57.0%) versus 22.2% (CI, 13.8% to 30.7%) after 1 year, and 49.1% (CI, 38.7% to 59.4%) versus 24.5% (CI, 15.6% to 33.4%) after 2 years. After adjustment for baseline characteristics, the hazard ratio for the post-thrombotic syndrome in the elastic stockings group compared with controls was 0.49 (CI, 0.29 to 0.84; P = 0.011). LIMITATIONS: This study lacked a double-blind design. CONCLUSIONS: Post-thrombotic sequelae develop in almost half of patients with proximal DVT. Below-knee compression elastic stockings reduce this rate by approximately 50%.