Denys-Drash syndrome is a rare human developmental disorder affecting the urogenital system and leading to renal failure, intersex disorders and Wilms' tumour. In this report, four individuals with this syndrome are described carrying germline point mutations in the Wilms' tumour suppressor gene, WT1. Three of these mutations were in the zinc finger domains of WT1. The fourth occurred within intron 9, preventing splicing at one of the alternatively chosen splice donor sites of exon 9 when assayed in vitro. These results provide genetic evidence for distinct functional roles of the WT1 isoforms in urogenital development.