A 71-year-old woman, 26 years post-menopause, presented with virilization. Facial hirsuties, non-hereditary frontal balding, voice changes, male escutcheon, and mild clitorimegaly accompanied a right adnexal mass. Blood hormone studies showed testosterone 430 ng/dL, FSH 118 mIU/mL, and LH 210 mIU/mL. By ultrasound examination, the cystic adnexal mass involved the right ovary. An 18-cm, 1300-g, unicameral mass with 1200 mL of clear serous fluid and with smooth inner and outer surfaces was removed from the right broad ligament. Intraoperative testosterone levels were as follows: peripheral vein 285 ng/dL, left ovarian vein 301 ng/dL, and right ovarian vein 1635 ng/dL; tumor cystic fluid was 3032 ng/dL. Peripheral vein testosterone was 15 ng/dL 3 days postoperatively. Histopathologically, the tumor was a serous cystadenoma. No evidence of stromal luteinization, hyperplasia, or inflammation was found, and other virilizing lesions were not encountered in either ovary or in other tissues. Epithelial cells constituting the tumor may have been the source of the excess testosterone in this unique case of virilizing serous cystadenoma.