Wilms' tumor is the most common pediatric renal neoplasm, but its occurrence in adults is very rare. In contrast to pediatric Wilms' tumor (PWT), very little is known about the pathogenesis of adult Wilms' tumor (AWT). Despite there currently being no morphological difference between AWT and PWT, a cytogenetic study has suggested that the pathogenesis of AWT might be different from that of PWT. Although dysregulation of the Wnt pathway has been implicated in PWT, its role in AWT has never been investigated. To investigate the role of dysregulation of the Wnt pathway in AWT, tumor samples from 4 AWTs and 19 PWTs were surveyed for subcellular localization of beta-catenin by immunohistochemistry and potential mutation of the beta-catenin gene by sequencing. Nuclear translocation of beta-catenin was found in one out of four cases of AWT, but none of them carried mutation of the beta-catenin gene. By comparison, nuclear translocation for beta-catenin and mutation of the beta-catenin gene were present in 53% (10/19) and 15.8% (3/19) of PWTs, respectively. Of the three mutations identified, we found a novel mutation combining a silent mutation (TCT to TCC, Ser37Ser) and an in-frame six-base-pair deletion (del GGTGCC, del Gly38Ala39). This report suggests that dysregulation of the Wnt pathway might also play a role in the pathogenesis of AWT.