Objective Clinicopathological characteristics and possible prognostic factors among women with endometrioid epithelial ovarian cancer (EEOC) with or without concurrent endometriosis were investigated. Study Design A search of medical charts at Peking Union Medical College Hospital from 2000 through 2012 identified patients with EEOC with or without endometriosis. Results Of 188 patients with EEOC, concurrent endometriosis was identified in 32 (17.0%). Patients with concurrent endometriosis were approximately 5 years younger, more likely to be premenopausal, more likely to have an early stage of EEOC, and less likely to have high-grade tumors compared to those without endometriosis. The univariate analysis showed that concurrent endometriosis was a significant prognostic factor for disease-free survival, but this association did not remain in the multivariate analysis. Conclusion Women with EEOC and concurrent endometriosis showed distinct characteristics and had longer disease-free survival when compared to those without endometriosis.