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Background: The current standard initial therapies for deep venous thrombosis are low-molecular-weight heparin and unfractionated heparin. In a dose-ranging study of patients with symptomatic deep venous thrombosis, fondaparinux had efficacy and a safety profile similar to those of low-molecular-weight heparin (dalteparin). Objective: To evaluate whether fondaparinux has efficacy and safety similar to those of enoxaparin in patients with deep venous thrombosis. Design: Randomized, double-blind study. Setting: 154 centers worldwide. Patients: 2205 patients with acute symptomatic deep venous thrombosis. Intervention: Fondaparinux, 7.5 mg (5.0 mg in patients weighing <50 kg and 10.0 mg in patients weighing >100 kg) subcutaneously once daily, or enoxaparin, 1 mg/kg of body weight, subcutaneously twice daily for at least 5 days and until vitamin 1< antagonists induced an international normalized ratio greater than 2.0. Measurements: The primary efficacy outcome was the 3-month incidence of symptomatic recurrent venous thromboembolic complications. The main safety outcomes were major bleeding during initial treatment and death. An independent, blinded committee adjudicated all outcomes. Results: 43 (3.9%) of 1098 patients randomly assigned to fondaparinux had recurrent thromboembolic events compared with 45 (4.1%) of 1107 patients randomly assigned to enoxaparin (absolute difference, -0.15 percentage point [95% CI, -1.8 to 1.5 percentage points]). Major bleeding occurred in 1.1% of patients receiving fond aparinux and 1.2% of patients receiving enoxaparin. Mortality rates were 3.8% and 3.0%, respectively. Limitations: Follow-up was incomplete in 0.4% of fondaparinux-treated patients and 1.0% of enoxaparin-treated patients. Conclusions: Once-daily subcutaneous fondaparinux was at least as effective (not inferior) and safe as twice-daily, body weight-adjusted enoxaparin in the initial treatment of patients with symptomatic deep venous thrombosis. [ABSTRACT FROM AUTHOR]