The karyotype of a cystic partially differentiated nephroblastoma (CPDN) was found to be 51, XY, +7, +8, +12, +13, +17. A review of the literature disclosed three other cytogenetically analyzed CPDNs. As in this case, they were all hyperdiploid. The only chromosomal anomaly common to all four cases was trisomy 12, suggesting this chromosome might have a pathogenetic role. Earlier reports had tentatively attributed this role to chromosome 8.