Primary tumors of the inferior vena cava (IVC) are rare. The most common primary malignancy is leiomyosarcoma (LMS), with a severe prognosis. Right-atrium (RA) involvement is seen in about 20 percent of cases. We report the case of a 53-year-old woman admitted to the intensive care unit because of progressive dyspnea with lower limb edema. Echocardiography demonstrated a large mass in the RA. At open-heart surgery, the tumor appeared located in the IVC infrahepatic segment, completely obstructing the lumen and extending into the suprahepatic portion and the RA. Histopathological examination concluded to LMS.