The genetics and associated abnormalities of Wilms' tumor are reviewed. Wilms' tumor is associated with several congenital syndromes such as WAGR (Wilms' tumor, aniridia, genitourinary malformation, mental retardation) syndrome, Denys-Drash syndrome, Beckwith-Wiedemann syndrome, etc. However, the association with such syndromes is relatively infrequent and accounts for less than 5% of all clinical patients with Wilms' tumor. WAGR syndrome and Denys-Drash syndrome are associated with loss of WT1 gene located in the chromosome 11p13, and BW syndrome is considered to be due to duplication of the paternal 11p15 allele (WT2). The association of Wilms' tumor with primary brain tumors in a daughter and a mother is also described.