A child with bilateral Wilms' tumors is reported. The left renal tumor showed nephroblastoma with several tissues of apparent mesenchymal derivation and tubules with diverse epithelial differentiation. The right-sided tumor showed the more familiar triphasic pattern of nephroblastoma. Initial percutaneous renal biopsy of the left tumor did not reveal nephroblastoma but showed tubules with various epithelia and mesenchymal elements, and led to a diagnosis of teratoma. The pathogenesis of this complex neoplasm is discussed, and argument is presented that primitive renal blastema may be capable of more diverse differentiation than has previously been realized. A comparison is drawn between this neoplasm and some cases of hepatoblastoma. Certain cases reported as renal teratoma may be similar in nature.