The authors studied genomic changes in unilateral Wilms' tumors by using WT33, a candidate cDNA for the tumor, and their correlation with histology. By Southern blot analysis, three cases of genomic deletions of both alleles were found in 25 tumors. The three tumors that showed genomic deletions were histologically classified as triphasic nephroblastic Wilms' tumor and one of them was associated with intralobar nephroblastomatosis and a rhabdomyomatous component. In one case, the WT1 gene was totally deleted, in another case, the 3' region of the gene was partially deleted, and in the last one, the deletion of DNA was intragenic. This is the first report of a comparison of genomic alteration with histopathology. These findings show new aspects of the role of the WT1 gene in the development of Wilms' tumor.