- Bone formation in the ovary, with the exception of developing in the setting of mature cystic teratoma, is exceedingly rare. A 46-year-old woman with a history of endometriosis and chronic pelvic pain underwent total abdominal hysterectomy and bilateral salpingo-oophorectomy. A 3 cm solid heavily calcified mass with a stony hard consistency was detected within the right ovary. Microscopic examination revealed extensive calcification of the right ovarian stroma with formation of abundant mature bone, adjacent to small foci of endometriosis. Endometriosis can be associated with ovarian ossification, forming an extensively calcified adnexal mass. Conservative treatment with close follow-up may be adequate in patients with a history of endometriosis who present with a small heavily calcified ovarian mass and wish to preserve their fertility.