Fifty patients with renal carcinoma were evaluated with real time ultrasonography: 15 tumors involved the renal vein (group Ia), 12 of them presented with caval tumor extension (group Ib); 35 tumors did not involve the renal veins (group II). In 3 cases, the renal vein could not be visualized. If the renal veins could be adequately delineated, evaluation of venous extension could be achieved in 100% cases (groups Ia and II). Nevertheless, caval tumor extension could be detected in only 70% cases (group Ib). This prospective study suggests that in the case of renal vein free of thrombus on ultrasonography, further investigations are not necessary to evaluate the venous extension of renal carcinoma. In case of inadequate delineation, or tumor involvement of the renal vein on ultrasonography, evaluation of inferior vena cava is mandatory, either by computed tomography, or inferior cavography.