Purpose: Mesothelin is a cell surface glycoprotein that is present on normal mesothelial cells and overexpressed in several cancers. In this study, we investigated the methylation/hypomethylation status in the promoter region of the mesothelin gene in gynecological tumors. Methods: Forty-four ovarian tumor specimens and 16 cases of uterine endometrial carcinoma, and normal tissue specimens were used. Monoclonal antibody (5B2) was employed for the immunohistochemical analysis. The methylation-sensitive single-nucleotide primer extension (Ms-SNuPE) technique was used to quantify the methylation/hypomethylation status at 20 CpG sites in the mesothelin promoter region. Results: Mesothelin was expressed in 100% of serous cystadenocarcinoma and 100% of serous borderline tumor of the ovary. None of the germ cell tumors and sexcord-stromal tumors was immunoreactive. Fifty percent of endometrial carcinoma was immunoreactive for mesothelin. The average methylation of CpG sites in ovarian tumors ranged from 6-56% (median: 31%) in mesothelin-positive and 13-79% (median: 43%) in mesothelin-negative samples. In endometrial tumors, the average methylation ranged from 5-52% (median: 28%) in mesothlin-positive and from 15-67% (median: 22%) in mesothlinnegative samples. A correlation was found between mesothelin expression and the average methylation/hypomethylation status as well as methylation/hypomethylation status at four of 20 CpG sites in ovarian samples. No correlation was found in endometrial samples. Conclusion: We detected diverse levels of methylation/hypomethylation at CpG sites in the mesothelin promoter region in ovarian and endometrial tumors. We speculate that, although methylation/hypomethylation changes may affect its transcription, other mechanisms may synergically operate in tissue-specific expression and tumor-related mesothelin overexpression.