To investigate the contribution of beta-catenin to the development of endometrial carcinoma, we searched for genetic alterations of the beta-catenin gene in primary endometrial carcinomas. Mutational analysis of exon 3 of the beta-catenin gene, encoding the serine/threonine residues for GSK-3 beta phosphorylation, was performed for 35 tumors. Nucleotide sequencing analysis revealed that 5 tumors (5/35, 14%) contained mutations (S33C, S37C, S37F, T41A) that altered potential GSK-3 beta phosphorylation sites. Each of the mutations resulted in the substitution of serine/threonine residues that have been implicated in the down-regulation of beta-catenin through phosphorylation by GSK-3 beta kinase. Furthermore, the incidence of beta-catenin mutations was significantly higher in early-onset (3 of 5) than that in late-onset tumors (2 of 30) (P = 0.014, Fisher's exact test). Replication error (RER)-positive phenotype was not detected in tumors with the beta-catenin gene mutation, although 10 of 35 tumors revealed RER. We performed immunohistochemistry of beta-catenin in 17 cases for which tissue samples were available. We confirmed accumulation of beta-catenin protein in both the nucleus and cytoplasm in 3 tumors, including two in which amino acid alterations had occurred at codon 33 and 37. The other case had no mutation in exon 3. Our results suggested that mutations at serine/threonine residues involved in phosphorylation by GSK-3 beta affected the stability of beta-catenin. Accumulation of mutant beta-catenin could contribute to the development of a subset of endometrial carcinomas, particularly those of the early-onset type.