The extrarenal location of Wilms' tumor is extremely rare. These tumors can arise from other neoplasms, generally teratomas, or they can present without any associated teratomatous elements. We encountered only 19 well-documented cases of isolated extrarenal nephroblastoma, and we describe three previously unreported patients with this disease. Two of the three children presented also with horseshoe kidneys, an association that may have clinical and embryologic significance. The presence of tumor cephalad to a horseshoe kidney is easier to explain when we consider that they originate from primitive mesodermal tissue, probably mesonephric rests, and not from metanephric remnants. Also, this association should alert physicians to the possible diagnosis of extrarenal nephroblastoma in patients with a retroperitoneal mass and horseshoe kidneys. These patients should be treated according to the same protocols as those for patients with intrarenal Wilms' tumor.