Endometriosis coexisting with a dermoid cyst of the ovary is extraordinarily rare, although these both benign conditions are said to be common in women in the reproductive age group. There are only two previous case reports, which is evident from our literature review from January 1960 through January 2010. Acute abdomen is one of the greatest diagnostic challenges and easily ignored by the clinicians to exclude the possibility of gynecologic illness. A 35-year-old woman was referred by the doctor in Family clinic. She experienced a three-day period of severe right lower abdominal pain and intermittent vomiting. Ultrasonography identified a bilocular, cystic, hypoechoic, and hyperechoic tumor, 7 cm×6 cm×6 cm in the right adnexal region. Laparoscopic cystectomy was performed under the impression of ovarian cyst with torsion or hemorrhage. The frozen section was benign and appendiceal status was adequate. Histopathologic examination described an ovarian cyst composed of endometrial-type lining with stroma cells (endometriosis) and benign terotoma tissue with plenty of skin appendages and sebaceous glands. We report this unusual and interesting ovarian mass to remind physicians that the usage of the Endobag after cystectomy, the benefits on minimizing operative time, spilled opportunity, and postoperative complications. Laparoscopic techniques for large ovarian masses might be considered. The experience of the surgeon is also very important to prevent misdiagnosis or complication. Further follow up is mandatory for this simultaneous finding of ovarian endometriosis with coincidental dermoid cyst as a separate pathology in single ovary of such a nature. It also presents a challenge to the clinicians and to the pathologists.