Wilms' tumour (WT) is a paediatric kidney tumour arising from the embryonal metanephrogenic blastema. Recent reports suggest that the expression of messenger RNA (mRNA) for insulin-like growth factor II (IGFII) is elevated in WT. Total cytoplasmic RNA was extracted from 11 sporadic WTs and analysed for IGFII mRNA using dot-blot hybridization. The level of IGFII mRNA expression varied greatly and not all tumours displayed enhanced IGFII expression. Two successive WT xenografts were established in nude mice. The original WT and first passage xenograft showed a blastematous histology, while the second passage xenograft showed epithelioid differentiation and tubule formation. Analysis of the expression patterns of these xenografts showed elevated IGFII expression in the primary undifferentiated tumour and the second differentiated xenograft, while the first undifferentiated xenograft failed to exhibit enhanced IGFII expression. These data show that elevated IGFII mRNA is not an essential component of the progression of WT and that WT tumourigenicity is independent of the level of IGFII expression. Therefore, IGFII overexpression in WT is most likely a tumour epiphenomenon.