The occurrence of genitourinary tumors in the relatives of a population-based series of 218 children diagnosed with renal tumors was investigated. Family data on 92% (176 of 192) of Wilms' tumor (WT) patients and 77% (20 of 26) of other renal tumor patients were obtained. In all, 21 genitourinary tumors in first-degree relatives in 19 families were ascertained, together with 30 such tumors in second-degree relatives. Ten families were diagnosed with multiple genitourinary tumors, although none of these manifested familial WT. It is proposed that a small proportion of families of children with renal tumors has a genetic predisposition to develop genitourinary tumors and that these tumors may represent further manifestations of the pleiotropic effects of the WT1 gene or of other genes involved in WT predisposition.