OBJECTIVE To present a case of a unilateral endometrioma with extremely elevated serum CA-125 levels. METHODS Case report. METHODS University hospital. METHODS A 25-year-old woman with left adnexal mass and extremely elevated serum CA-125 level, 7,900 U/mL, underwent laparoscopy. There were unilateral endometrioma and stage IV endometriosis. METHODS Laparoscopic excision of the endometrioma, unilateral salpingectomy, adhesiolysis, ablation of endometriotic foci, and partial omentectomy were performed, and histopathologic results confirmed the diagnosis of endometriosis. METHODS Extremely elevated serum CA-125 levels and unruptured ovarian endometrioma. RESULTS On the second, sixth, and 13th days of the menstrual period, serum CA-125 levels were 7,900 U/mL, 1,577 U/mL, and 627 U/mL, respectively. On the third postoperative day serum CA-125 level was 56 U/mL. CONCLUSIONS Ovarian endometrioma and advanced endometriosis may be associated with extremely elevated serum CA-125 levels. For this reason ovarian endometrioma should be considered with respect to differential diagnosis of reproductive-age women presenting with an ovarian mass even if it resembles an ovarian malignancy.