Metanephric adenoma (MA) is a renal tumor that is generally detected in adults and occasionally in children. These tumors usually behave in a benign fashion. Although the histogenesis of MA is unclear, a morphologic similarity to Wilms' tumor (WT) complex exists. Six cases of MA, five cases of childhood WT (CWT), two cases of adult WT (AWT), and four cases of treated MWT and/or nephrogenic rests (MWT/NR), with paraffin blocks available for use, were retrieved from the surgical pathology files of the Mayo Clinic. Clinical information was extracted from the medical record. Immunoperoxidase stains for WT1, AE1, CK7, CD57, CD56, and desmin were performed on paraffin sections from all cases. All six cases of MA were strongly and diffusely positive with antibodies to WT1 and CD57 and focally positive with antibodies to CK7. Three cases showed focal faint staining in <5% of the cells with keratin AE1. Stains for CD56 and desmin were negative. All seven cases of WT, including five CWT and two AWT, were strongly and diffusely positive with WT1 in the blastema and epithelium but showed only weak focal positivity in stromal cells. Six cases were diffusely positive for CD56 and one case showed focal positivity. Keratin AE1 was positive in one case of AWT and focally positive in the other AWT. The blastema of all cases of WT were negative for desmin, CK7, and CD57, although staining for keratin AE1, CD56, and CD57 was seen in maturing tubules of CWT cases. Of the five CWT cases, two had associated NR and two showed maturing WT after treatment. The areas of NR and maturing WT were histologically similar to MA and were composed of small tubules with uniform nuclei with no mitotic activity, scant cytoplasm, and focal calcifications. All four cases of maturing WT/NR were positive for WT1 and focally positive for CD57, CK7, and AE1. Stains for CD56 and desmin were negative except in foci of residual blastema, which stained for CD56 but lacked CD57 and CK7 staining. Five cases each of renal cell carcinoma, papillary renal cell carcinoma, and oncocytoma were negative for WT1. Two of five cases of chromophobe carcinoma showed very weak staining present in <10% of tumor nuclei. Metanephric adenoma is histogenetically related to WT and is morphologically and immunophenotypically identical to maturing WT and nephrogenic rests.