The presence of tumor thrombus secondary to inferior vena caval extension from renal carcinoma carries the threat of pulmonary tumor embolus. In theory, safe prophylaxis could be accomplished by placement of a Greenfield filter in the suprarenal vena cava, which has been accomplished without complication. We treated 6 patients with renal call carcinoma and extensive tumor thrombus of the vena cava with suprarenal filter placement as an adjunct to thrombectomy and nephrectomy. Clinically all 6 patients have done well. However, the over-all rate of vena caval thrombosis or occlusion associated with infrarenal filter placement is 3 to 5%. To investigate the potential risk to renal function if a vena caval occlusion occurred above a solitary kidney shortly after unilateral nephrectomy, we performed suprarenal inferior vena caval ligations after unilateral nephrectomy in 10 dogs. A total of 6 dogs suffered persistent loss of renal function and 3 of these 6 died of uremia. Of 4 dogs who underwent suprarenal inferior vena caval ligation only 1 (25%) had persistent compromise of renal function. A total of 2 dogs underwent unilateral nephrectomy only without compromise of normal renal function. We conclude that the risk of total vena caval occlusion after suprarenal Greenfield filter placement is small. However, should it occur in the setting of recent nephrectomy there is potential for significant renal morbidity. In selected patients this risk may be offset by the potential benefits that the filter offers in terms of protection against tumor and/or bland pulmonary embolus. Further clinical experience will be needed to strengthen and clarify the indications and benefits of preoperative or intraoperative filter placement as reported.