Both nephroblastomatosis and deletions of the short arm of chromosome 11 (11p-) have been associated independently with Wilms' tumor. The finding of 11p- in a specimen of nodular renal blastema in the currently described patient represents a previously unknown association with this chromosomal lesion. The possibility that 11p- produced an abnormal renal substrate (nephroblastomatosis), upon which the action of a second postzygotic genetic alteration led to Wilms' tumor, is considered. It is suggested that, in the present case, tumorigenesis may have been the result of two postzygotic events, one of which may have been postnatal. Recent cytogenetic observations in both Wilms' tumor and retinoblastoma support such an hypothesis.