In nephrectomy specimens from a series of 24 Wilms' tumors three different types of neoplastic lesion were found in the uninvolved kidney parenchyma: nodular renal blastema, Wilms' tumorlet and metanephric hamartomas. The histologic features of the first two abnormalities are presented. They show the following characteristics: Being found in patients of younger age they occur mostly bilateral. In older children they are associated to Wilms' tumor. They are related, as this is the case with Wilms' tumors, to certain congenital malformation syndromes. Nodular blastema has been also observed in association with Wilms' tumor in siblings. These findings suggest a genetic relationship between nodular renal blastema-nephroblastomatosis complex and Wilms' tumor. Furthermore, such lesions may represent potential progenitor stages of most nephroblastomas. The clinical implications hereof are discussed.