Wilms' tumour is a paediatric kidney cancer which, in a substantial number of cases, has been associated with a genetic predisposition. Susceptibility to Wilms' tumour can be manifested by the presence of bilateral tumours, and in rare cases by a family history of this tumour or by associated congenital malformations. Like retinoblastoma, Wilms' tumour has been postulated to result from the inactivation of a tumour suppressor gene, although genetic studies implicate more than a single genetic locus. The recent isolation of the WT1 gene, which maps to chromosome 11, band p13, has provided the first molecular clue to Wilms' tumorigenesis. WT1 is specifically inactivated in a number of Wilms' tumours, and mutations have been found in the germline of susceptible individuals. This gene appears to encode a transcription factor with complex alternative splices, whose expression is strictly regulated in the developing kidney. Functional studies will be required to elucidate the role of WT1 in normal kidney development and in tumorigenesis.