Somatic alterations in the genome are found in many human tumours. Chromosome rearrangements or base substitutions that activate cellular oncogenes appear to act dominantly. In contrast, recessive alleles apparently contribute to childhood retinoblastoma, as homozygosity (or hemizygosity ) for chromosome 13 is often established in tumours, by either mitotic nondisjunction or recombination. Parallels exist between retinoblastoma and childhood Wilms' tumour (WT). Retinoblastoma is often inherited and accompanied by a deletion of chromosome 13 (band q14), while WT is occasionally associated with aniridia and deletion of chromosome 11 band p13. Most Wilms' tumours are sporadic and not accompanied by these findings, although interstitial deletion of chromosome 11 in tumour, but not normal, cells has been reported. In view of these parallels, we compared constitutional and tumour DNAs from WT patients by using chromosome 11p DNA probes. We report here that although heterozygosity in constitutional DNAs was often preserved in tumour DNAs, one case developed homozygosity for chromosome 11p markers in tumour cells, implying the involvement of chromosomal events in revealing a recessive WT locus. This observation suggests the action of such general mechanisms in a tumour other than retinoblastoma.