A patient with diabetes and breast cancer in whom virilization was caused by a testosterone-producing mature cystic teratoma containing a Brenner tumor.

A 74-year-old woman with impaired glucose tolerance exhibited virilization. An examination of various hormone levels showed normal pituitary hormone and adrenal hormone levels. However, the patient's blood testosterone level was remarkably high and was suspected of having caused the virilization. An abdominal computed tomography revealed a multilocular cystoma in the left ovary with the features of a mature cystic teratoma. In addition, a chest computed tomography revealed the presence of a mass in the left breast that was subsequently diagnosed as breast cancer based on the results of a biopsy. After the simultaneous surgical resection of the ovarian and breast tumors, her blood testosterone level decreased. The mature cystic teratoma containing a Brenner tumor was considered to be responsible for the high testosterone level because of the presence of androgenic enzymes. This case is extremely rare, but the case was accurately diagnosed through a comprehensive analysis.