Extrarenal Wilms' tumor is a rare entity usually seen as a mass in the retroperitoneal area. It may surround and distort otherwise normal kidneys and ureters. Like many other abdominal masses, its true nature remains uncertain until microscopic examination has been performed after surgery. The tumors appear to behave clinically like intrarenal Wilms' tumors, though in the cases reviewed there was a relatively higher incidence in older patients. The prognosis of extrarenal Wilms' tumor is difficult to evaluate due to the small numbers of cases reported and the differing tumor therapy used. However, those cases treated most recently appear to have a good response to therapy.