WT1, the Wilms' tumour suppressor gene located at chromosome 11p13, plays an important role in the development of the kidney. It is mutated in 10 per cent of Wilms' tumours (WTs) and their putative precursors called nephrogenic rests (NRs). A sensitive immunohistochemical technique was established to localize the WT1 gene product in archival normal tissues and paediatric renal tumour samples. Strong nuclear signal was seen in the various sites such as the kidneys, gonads, and decidua. Nuclear immunoreactivity of variable intensity was also seen in the skeletal muscle, smooth muscle of urinary bladder, ureter, and arteries. All 28 WTs (including the anaplastic variant) showed the WT1 gene product in a varying proportion of the blastema, epithelium, and stroma. The anaplastic nuclei in six WTs showed an intensity of staining comparable to their adjacent favourable histology counterparts. All the intralobar and perilobar types of NR demonstrated the WT1 gene product. All three malignant rhabdoid tumours were positive, while three of four mesoblastic nephromas and a clear cell sarcoma were negative. These findings provide an insight into the interrelationships of these renal tumours.