Nineteen patients with histologically proved tumor thrombi of the inferior vena cava (IVC) secondary to abdominal neoplasms were studied with the use of ultrasonography (US) and computed tomography (CT). The primary neoplasms were renal cell carcinoma (13 cases), adrenal tumors (two cases), retroperitoneal tumors (two cases), and hepatic tumors (two cases). A positive diagnosis of tumor thrombus was achieved by both methods. The cranial extent of the tumor thrombus was better demonstrated by US studies, which showed echogenic endoluminal material within an enlarged IVC with a bulging anterior wall. On CT scans the tumor thrombus usually appeared as an endoluminal filling defect surrounded by a rim of contrast material. Tumor thrombus was better outlined by CT, particularly when it extended beyond the limits of the IVC wall, as in the retroperitoneal tumors. Neither method could accurately be used to predict IVC wall infiltration when the tumor thrombus remained within the confines of the IVC, nor could either method differentiate tumor from nontumor thrombi. US and CT are complementary in the preoperative assessment of tumor thrombus, and their use obviates the need for venacavography in many cases.