Although malignant neoplasm of the venous system is rare, leiomyosarcoma of large veins (predominantly the inferior vena cava) constitutes more than 75% of this condition. Symptoms are subject to tumor location and stenosis or obstruction of the venous system. This report describes the cases of two patients with leiomyosarcoma originating in large veins. Venography is extremely important for diagnosis, however computerized axial tomography with dye enhancement is helpful to depict the neoplastic nature of this condition. Excision and replacement with autogenous vein or artery will provide better long-term patency.