Selected studies of Wilms' tumor and related renal neoplasms in children, which have been based on the National Wilms' Tumor Study Pathology Center collection of more than 2,600 renal tumors of childhood, are reviewed. The purpose of the review is to illustrate the value of the collaborative approach to uncommon pathologic specimens and to distinguish several tumors often confused with Wilms' tumor, including renal adenocarcinoma, renal teratoma, and renal neurogenic tumors. The unfavorable prognostic significance of anaplastic cells in Wilms' tumor is emphasized. Two recently described clinicopathologic entities--clear cell sarcoma of kidney and malignant rhabdoid tumor of kidney--and morphologic variants often confused with Wilms' tumor or congenital mesoblastic nephroma of infancy are discussed. Clear cell sarcoma of kidney and malignant rhabdoid tumor of kidney are, apparently, distinctive neoplasms, not Wilms' tumor variants.