In 16 patients with surgical confirmation of inferior vena cava thrombi from renal carcinoma, magnetic resonance imaging (MRI) and computed tomography (CT) were compared to assess the ability of non-invasive, cross-sectional imaging techniques to detect tumour thrombus, the level of its extension, and vessel wall invasion. MRI accurately detected tumour thrombus in the inferior vena cava in all 16 cases (CT in 14) and demonstrated the cephalad extent of tumour thrombi in 15 of 16 (CT in 11). The level of extension was more easily seen on MRI, as was the presence of extension into the hepatic veins--a finding not detected by CT. Tumour invasion of the wall of the inferior vena cava was correctly demonstrated in 7 cases on MRI and in 1 case on CT. In 8 of 16 patients, gradient recall acquisition in steady state (GRASS imaging) was performed in addition to conventional spin echo sequences and it correctly identified the composition of thrombus in all: tumour in 6 patients, blood clot in 1 and both tumour and blood clot in the remaining patient. MRI and CT are excellent for detection of tumour but MRI is superior in the evaluation of vascular extension. In addition, the use of GRASS imaging allows differentiation of tumour from blood thrombus.