The majority of tumors which involve the inferior vena cava (IVC) or iliac veins are malignant. IVC and iliac vein tumors are divided into two types: primary and secondary. Some surgeons arbitrarily call the region around the renal vein–caval confluence the pararenal IVC segment. The most common location of primary venous leiomyosarcoma (PVL) is the IVC, with the suprarenal segment involved most often. PVL is hard to distinguish from other retroperitoneal sarcomas if it is large and invades through the IVC wall. Retroperitoneal sarcoma is the most common secondary tumor to extrinsically compress and invade the infrarenal IVC or iliac veins. Patients with IVC or iliac vein malignancies present with symptoms and signs related to the tumor or its metastases, but rarely due to venous obstruction. Symptomatic IVC obstruction occurs from acute thrombosis when venous collaterals are poorly developed or when the venous hypertension caused by progressive IVC or iliac vein obstruction exceeds the capacity of the collaterals.