The immunohistochemical expression pattern of beta-catenin has been correlated with beta-catenin gene mutations, clinicopathological features, and disease outcome in 69 stage I and II ovarian carcinomas. beta-Catenin expression was localized in the nuclei, in addition to the cytoplasm and membrane, in 11 tumors (16%): nine endometrioid carcinomas with widespread nuclear expression and two serous carcinomas with focal nuclear expression. The remaining 58 carcinomas (84%) only had membranous beta-catenin expression. All but one of the endometrioid carcinomas with nuclear beta-catenin expression had considerable squamous metaplasia, and five of these cases had large areas of endometrioid tumor of low malignant potential. In addition, beta-catenin nuclear expression was observed in atypical epithelial cells in endometriotic glands adjacent to an endometrioid carcinoma. Sequencing was performed on 25 tumors and corresponding normal tissue: all 13 endometrioid tumors as well as 12 carcinomas of other histological types (four serous, two clear cell, two mucinous, and two mixed). There were oncogenic mutations in the phosphorylation sequence for GSK-3beta in exon 3 of the beta-catenin gene in seven endometrioid carcinomas with beta-catenin nuclear expression. Three mutations affected codon 32 (D32G, D32Y, and D32Y), one affected codon 33 (S33C), two affected codon 37 (S37C and S37F), and one affected codon 41 (T41A). No mutations were observed in the other 18 carcinomas analyzed, comprising two endometrioid and two serous carcinomas with beta-catenin nuclear expression, and 14 carcinomas of different histological types with only membranous expression. In the univariate and multivariate survival analyses, beta-catenin nuclear expression was selected as an indicator of good prognosis, because no patient whose tumor expressed beta-catenin in the nuclei showed relapses or died, in contrast to the 19 relapses and deaths among patients with tumors that only had beta-catenin membranous expression, including three of the four patients with endometrioid carcinomas. Oncogenic beta-catenin mutation is characteristic of a group of endometrioid carcinomas with a good prognosis, most of which originate from previous benign or borderline lesions. Endometrioid carcinomas with exclusively membranous expression of beta-catenin seem to represent a different subgroup of carcinomas that probably have a worse prognosis. In early-stage ovarian cancer, determination of the beta-catenin expression pattern could prove to be a useful marker for selecting low-risk patients.