Wilms' tumor (WT) is the most common childhood renal malignancy. Although several genetic loci such as the WT1 gene have been known to relate to the biology of WT, the cause of the tumor is complex and the implicated molecular pathways are largely unknown. The beta-catenin gene encodes a protein playing an important role in the Wnt signaling pathway, and its mutations that abrogate specific serine/threonine phosphorylation sites and express oncogenic effect have been found in a variety of tumors. Implication of beta-catenin mutations in WT was investigated in 24 tumors collected from 20 WT patients. One patient had a total of five multiple tumors simultaneously in the bilateral kidneys. Exon 3 and its flanking regions encompassing mutational hot spots of the gene were examined by PCR-based methods. Samples indicating to harbor mutations were further analyzed by sequencing. Six tumors (6/24, 25%) from 4 patients (4/20, 20%) were confirmed to have mutations in heterozygous status. All the mutations, including five different types, were uniformly observed at codon 45 (Ser). Three mutations, Ser45Phe (TCT --> TTT), Ser45Tyr (TCT --> TAT), and Delta45 (deletion of TCT), were found in 3 of 19 unilateral WTs. Other three mutations were detected in three of five multiple tumors developed in the bilateral WT patient; a mutation of Delta45 in one of two tumors in the right kidney, and Ser45Cys (TCT --> TGT) and Ser45Pro (TCT --> CCT) in two of three tumors in the left kidney. Frequent beta-catenin mutations preferentially occurring at codon 45 most likely indicate special importance of this codon for the development of WT and existence of an underlying mechanism causing such a tissue-specific mutational pattern.