Nephroblastoma is commonly a chance finding in a series of malformation syndromes. With a frequency of 4-8%, urogenital malformations are amongst the most common congenital anomalies in Wilms' tumor patients. For various congenital anomalies, there is a genetic predisposition and thus an increased risk of developing Wilms' tumor. The WT1 gene product (WT1 protein) usually regulates genes that play an important role in normal kidney development and in maintaining kidney function. If this gene has a functional change, then this role is no longer possible. The consequence is the development of nephrotic syndrome and possibly of a Wilms' tumor.