WILMS' tumour (WT), aniridia, genitourinary abnormalities and mental retardation form a symptom group (WAGR syndrome) associated with hemizygous deletions of DNA in chromosome band 11p13 (refs 1,2). However, it has not been clear whether hemizygosity at a single locus contributes to more than one phenotype. The tumour suppressor gene for Wilms' tumour, WT1, has been characterized^3,4: it is expressed at high levels in the glomeruli of the kidney⁵, as well as the gonadal ridge of the developing gonad⁵, the Sertoli cells of the testis⁶ and the epithelial and granulosa cells of the ovary⁶, suggesting a developmental role in the genital system in addition to the kidney. We now report constitutional mutations within the WT1 genes of two individuals with a combination of WT and genital abnormalities as evidence of a role for a recessive oncogene in mammalian development.