We have examined a panel of 21 sporadic Wilms' tumours for rearrangements in the Wilms' tumour suppressor gene, WT1. In one tumour with specific allele loss in chromosome 11p13, a homozygous deletion in the 3' end of the gene, encompassing exon 10 and the 3' untranslated region, was identified. High levels of a truncated WT1 transcript, predicted to encode a polypeptide missing the fourth zinc finger were expressed in this tumour. All other samples showed normal patterns of digestion on Southern blots. This observation confirms previous findings that large deletions in the gene occur infrequently in sporadic Wilms' tumours and that the zinc-finger region of the encoded polypeptide is critical for correct functioning of the gene.