Perilobar nephrogenic rests (NR) are precursor lesions that may display genetic changes similar to their associated Wilms tumor (WT). Two patients presented with WT, both with perilobar NR and 1 with bilateral, multifocal metachronous WT. Both patients' WT displayed monosomy 22 as the predominant cytogenetic change, and the constitutional cytogenetic analysis was normal. The purpose of our study was to identify at what stage in the morphologic progression from NR to WT the monosomy 22 occurred by using a fluorescent in situ hybridization probe for chromosome 22 in the subtypes of perilobar NR and WT present in both cases. Section and core fluorescent in situ hybridization with a chromosome 22 probe was performed on formalin-fixed, paraffin-embedded tissues containing WT and perilobar NR. We also performed fluorescent-based microsatellite analysis on some of the WT in the bilateral case to determine whether there was a preferential loss of the same allele of chromosome 22. Sclerotic and dormant perilobar NR showed a rate of monosomy 22 of only approximately 30%, but this increased to approximately 50% in hyperplastic and adenomatous NR. Monosomy 22 was present in 60%-80% of nuclei in WT. Microsatellite analysis showed loss of homozygosity, with preferential loss of the same allele of chromosome 22 in the tumors examined. There are differences in the rate of detection of monosomy 22 in perilobar NR and WT, suggesting loss of chromosome 22 in the progression of perilobar NR to WT in a subset of tumors.