Wilms' tumor invades the renal vein in 12% of patients and extends into the inferior vena cava in another 6%. In a few cases, the tumor propagates into the right side of the heart. Since patients with Wilms' tumor seldom require renal arteriography, and because vena cava involvement may be silent, venous invasion may go undetected until the time of radical nephrectomy. A preoperative vena cavogram is recommended, and if a thrombus is found, a right heart angiogram is needed to determine the superior extent of thrombus. An understanding of the collateral venous circulation of the kidneys and of the physiological effects of malignant or iatrogenic obstruction of the inferior vena cava are essential to formulating the operative approach. Various operative maneuvers, intraoperative studies, and surgical aids (including cardiopulmonary bypass and hypothermia) have made forms of intracaval Wilms' tumor amenable to excision.