Objective: Extrauterine formation of decidua of stromal cells has been well described, particularly in the cervix and ovary. The apparent hormonal mechanisn of this phenomenon suggestes a relationship to endometriosis. Whether formation of ectopic decidua represents a marked progestational response of endometriosis or an independent peritoneal-stromal reaction to pregnancy is unclear. This study was designed to determine the prevalence of deciduosis in the patients whose fertility were proven. Design: Prospective study of patients who had undergone cesarean delivery without history of endometriosis. Materials and Methods: The study was performed in 179 full tenn pregnant women. During the cesarean section, the pelvic organs were thoroughly investigated and the biopsies were collected at the lesions suspicious endometriosis. And then microscopic examination of removed tissues were done. Results: Of the 179 patients who underwent cesarean delivery, 48 women (26.8%) had the lesions suspicious endometriosis such as adhesion, pigmented spots. The ovary was the most frequently ocurred site (79.2%). Microscopically, decidual cells were observed in 34 cases (70.8%) of 48 biopsed patients. Conclusion: Endometriosis has been known to be associated with subfertility. Our observations found the prevalence of deciduosis was 19.0% (34/179) in tenn pregnant women whose fertilites were proven. We suggests that the deciduosis maya manifestation of endometriosis during pregnancy. However, further follow up study should be done to confirm this clinicopathologic process.