OBJECTIVE: To investigate the prevalence, the distribution of deciduosis, and the relationship with endometriosis in fertile women during Cesarean delivery. METHODS: In this study, pelvic tissues suspicious for ectopic deciduas were taken for biopsy during Cesarean section from 154 parturients of full term pregnancy from January 1990 to December 2003. And then those patients were followed up till April 2008. RESULTS: Tissues from 94 parturients (94/154, 61%) were evaluated histopathologically, and ectopic decidua was observed in 70.2% (66/94). Ectopic sites were ovaries only (65/94, 69.1%), ovaries and uterine serosa (12/94, 12.8%), uterine serosa only (9/94, 9.6%), and pelvic serosa. Twenty seven (27/66, 40.9%) parturients had past history of diagnosis and treatments for endometriosis. We have tried to connect 39 (39/66, 59.1%) patients who had never been diagnosed for endometriosis but pathologically confirmed for deciduosis, and 18 patients were able to contact by phone. Twelve patients (12/18, 66.6%) showed no symptoms of endometriosis and had not received any treatments for endometriosis. CONCLUSION: We can conclude that most of incidental cases confirmed pathologically for deciduosis during pregnancy do not symptomatically progress.