Benign epithelial nephroblastoma. A contribution to its histogenesis.

The occurrence of a malignant Wilms'tumor in the right kidney and of a benign epithelial nephroblastoma in the left kidney of a 5-year-old girl is reported. Both kidneys contained foci of persistent well differentiated blastema. In the left kidney a direct transformation of the primitive metanephric epithelium into a benign nephroblastoma is shown. This finding suggests an origin of epithelial nephroblastoma from persistent nephrogenic tissue and indicates that the former represents the benign counterpart of the malignant Wilms'tumor.