Hypoplasia of the inferior vena cava can be revealed by a deep venous thrombosis of the lower limbs. Associated precipitating factors or clotting defects leading to thrombosis are frequently observed. A case of bilateral iliac veins thrombosis occurring after a motor vehicle accident with seat belt injury is reported, revealing hypoplasia of the inferior vena cava. This young man was totally asymptomatic up to the crash, and did not have coagulation abnormalities. The patient had a very good outcome after anticoagulant treatment with complete regression of venous thromboses. Hypoplasia of the inferior vena cava was a predisposing anatomic abnormality that led to thrombosis, but seat belt trauma was probably the precipitating factor. This observation should be kept in mind in the evaluation of a deep venous lower limb thrombosis.