To establish whether loss of heterozygosity (LOH) for chromosome 16q in Wilms' tumours confers an adverse prognosis, DNA from 40 Wilms' tumour/normal pairs were analysed using highly polymorphic microsatellite markers along the length of 16q. Fifteen per cent of tumours showed LOH for 16q. Although the common region of allele loss spanned the 16q24-qter region, a second distinct region of LOH was identified in 16q21. Five out of six tumours showing LOH were either (1) high stage or (2) low stage with unfavourable histology. In addition, there was a higher mortality rate in patients showing LOH for 16q than those that did not. These data strongly support the suggestion that LOH for 16q is associated with an adverse prognosis.