Individuals with constitutional, heterozygous deletions of chromosome region 11p13 are predisposed to the development of Wilms' tumour, indicating the site of the tumour predisposition gene. The WT1 gene is a candidate for this cancer predisposition gene. If this gene is truly involved in tumorigenesis it would be expected to be mutant in tumour tissue from patients with 11p13 deletions. We have used single-stranded conformation polymorphism (SSCP) and polymerase chain reaction sequencing to test this hypothesis in an exon-by-exon analysis of the gene. Four tumours were analysed, two of which were from unilaterally affected individuals and two from a bilaterally affected patient. SSCP analysis identified mutations in the two unilateral tumours which, on sequencing, were shown to involve a 10-bp insertion in exon 7 and a single base pair change in exon 8. Both mutations result in the generation of premature stop codons and the predicted proteins would lack part of the zinc finger motif. Despite complete sequencing of the WT1 gene in both of the bilateral tumours, no mutations were identified. These results possibly suggest that WT1 may not be involved in tumorigenesis in all tumours. All four tumours retained heterozygosity in the 11p15 region, making it unlikely that a second recessive oncogene in this region was involved in tumorigenicity.