Denys-Drash syndrome is a rare human condition in which severe urogenital aberrations result in renal failure, pseudohermaphroditism, and Wilms' tumor (nephroblastoma). To investigate its possible role, we have analyzed the coding exons of the Wilms' tumor suppressor gene (WT1) for germline mutations. In ten independent cases of Denys-Drash syndrome, point mutations in the zinc finger domains of one WT1 gene copy were found. Nine of these mutations are found within exon 9 (zinc finger III); the remaining mutation is in exon 8 (zinc finger II). These mutations directly affect DNA sequence recognition. In two families analyzed, the mutations were shown to arise de novo. Wilms' tumors from three individuals and one juvenile granulosa cell tumor demonstrate reduction to homozygosity for the mutated WT1 allele. Our results provide evidence of a direct role for WT1 in Denys-Drash syndrome and thus urogenital system development.