Renal cell carcinoma has a propensity to extend as tumor thrombus into the renal vein and inferior vena cava (IVC). The preoperative assessment for the presence and extent of renal vein and IVC tumor thrombus is important for planning appropriate surgical resection. Imaging procedures [CT, ultrasound (US), MR, venacavography] were correlated with surgical findings and pathology in 431 consecutive patients who had a radical nephrectomy for renal cell carcinoma. Ninety-nine (23%) patients had tumor thrombus extending at least into the main renal vein. Of these, 29 had tumor thrombus extending within the IVC. Patients were classified into two groups based on the surgical extent of tumor thrombus. Group A patients had no tumor thrombus or had tumor thrombus only in the renal vein proximal to the site of surgical ligation. Group B patients had tumor thrombus that extended to or beyond the distal renal vein at the site of surgical ligation. Forty-one patients had Group B tumor thrombus. Group B tumor thrombus was not seen in a renal cell carcinoma that was smaller than 4.5 cm. The sensitivity of CT for detecting Group B tumor thrombus was 79% and that of US was 68%. However, a much higher percentage of US examinations were technically indeterminate. In the patients who had either MR or venacavography, both imaging procedures were 100% sensitive for detecting group B tumor thrombus. Magnetic resonance imaging and venacavography appear to be the most sensitive means of identifying tumor thrombus.