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Objective— Deep vein thrombosis (DVT) and pulmonary embolism are frequent causes of morbidity and mortality. The goal of our study was to determine whether plasma high-density lipoprotein (HDL), which inversely correlates with the risk of cardiovascular events, affects DVT. Methods and Results— Using a murine DVT model of inferior vena cava stenosis, we demonstrated that deficiency of the HDL receptor, scavenger receptor class B type I (SR-BI), promotes venous thrombosis. As SR-BI −/− mice have increased plasma cholesterol levels and abnormal HDL particles, we tested SR-BI −/− mice with an SR-BI liver transgene that normalizes both parameters. These mice also exhibited increased susceptibility to DVT, indicating a protective role of extrahepatic SR-BI. Mice lacking the major HDL apolipoprotein apoA-I or endothelial nitric oxide synthase (eNOS) (a downstream target of endothelial SR-BI signaling) also had a prothrombotic phenotype. Intravenous infusion of human apoA-I, an HDL component and SR-BI ligend, prevented DVT in wild-type but not SR-BI −/− or eNOS −/− mice, suggesting that its effect is mediated by SR-BI and eNOS. Intravenous apoA-I infusion abolished histamine-induced platelet–endothelial interactions, which are important for DVT initiation. Conclusion— An apoA-I (HDL)–SR-BI–eNOS axis is highly protective in DVT and may provide new targets for prophylaxis and treatment of venous thrombosis.