A nephroblastoma (Wilms' tumor) with morphological, histochemical, immunohistochemical and ultrastructural evidence of neuroendocrine differentiation is described. Whereas areas of neural differentiation and occasional argyrophilic cells in cases of Wilms' tumor have been previously reported, the unique characteristic in this case was the extent of the neuroendocrine differentiation, as shown by a strong Grimelius stain of over 90% of the blastematous cells. Immunoperoxidase studies employing antibodies to neuron-specific enolase and ultrastructural data were also in favor of the neuroendocrine differentiation and suggested the existence, in addition to the already reported variant of Wilms' tumor with neural differentiation, of a neuroendocrine variant which may be part of the histologic spectrum of this neoplasm.