Twenty-three Wilms' tumors with various histologic patterns and normal human metanephroi from approximately 17-week identical twin fetuses were studied by electron microscopy. Cells of the blastemal cap in the normal-developing metanephroi showed characteristics of epithelioid cells. Frequently, cilia formation was seen in normal-developing proximal tubules. There were remarkable similarities in ultrastructure between normal-developing metanephron units and epithelial components of Wilms' tumors at various stages of differentiation. The neoplastic epithelial cells could differentiate into tubular structures showing segmentation. There were, however, distinctive ultrastructural aberrations in Wilms' tumors. “Pseudoglomeruli” in the tumors were composed entirely of epithelial cell masses resting on basement membranes, and neither capillary endothelial cells nor mesangial cells were present. There seemed to be asynchronous development among individual cytoplasmic organelles and also between cytoplasm and nuclei in tumor cells. Apical tubules and related systems were seldom seen, and no complicated basilar infoldings were observed in differentiated epithelial cells in which other cytoplasmic organelles were well developed. Cilia formation was found in 8 of the 23 tumors and may be an expression of the tumors' embryonal character. Our observations suggest that all neoplastic cells originating from blastemal cap cells differentiate only into epithelial components of the tumor.