In 4 patients with hypernephroma and extension of the tumor thrombus into the renal vein, there has been a significant degree of proteinuria. We believe that the proteinuria in association with a renal mass suggests a tumor thrombus in the renal vein and warrants the preoperative inferior venacavogram. After a larger number of patients have been studied, it may become clear that patients who do not have proteinuria do not have renal vein or vena caval involvement. If this proves to be the case, these patients may be selectively spared inferior venacavography. For the present time in patients with hypernephroma, we can only recommend that the index of suspicion for renal vein involvement with tumor thrombus should remain high if the patient has significant proteinuria.