This report defines the incidence and determines the etiology of renal failure (RF) in patients undergoing treatment for Wilms' tumor (W). The database of the National Wilms' Tumor Study (NWS) was searched to identify all children reported to have developed chronic renal failure. There were 55 patients found to have RF. Of these, 39 patients had bilateral tumors, 15 with unilateral disease and one with a W in a solitary kidney. The median interval from diagnosis to the onset of renal failure was 21 months. The incidence of RF in bilateral W was 16.4% for NWS-1 & -2,9.9% for NWTS-3, and 3.8% for NWTS-4. The incidence of RF in unilateral WT remained stable. The most common eti-ologies of RF were: bilateral nephrectomy for persistent or recurrent tumor (24 pts), Drash syndrome (12 pts), progressive tumor in the remaining kidney (5 pts), radiation nephritis (6 pts), and other causes (5 pts). The etiology of renal failure was not reported in three children. Children with unilateral WT and a normal contralateral kidney have a very low incidence of RF, and this review does not support a recommendation for parenchymal sparing procedures in these patients. Children with bilateral W are at risk for the development of RF, and parenchymal sparing procedures are warranted.