Carbohydate antigen 125 (CA-125) is a type of cell surface glycoproteins present in more than 80% of non-mucinous epithelial ovarian carcinomas; however, benign gynecologic conditions commonly cause a smaller increase in CA-125 level. This report presents the details regarding a 44-year-old woman with extremely high serum CA-125 level and ascites. She complained of having abdominal pain and abdominal distension. Her serum CA-125 level had been markedly elevated (>10,000 IU/ml) and computed tomograpgy (CT) revealed an ovarian tumor and massive ascites. The cytological analysis showed no evidence of malignancy, however, the positron emission CT (PET-CT) scan suggested ovarian malignancy with peritoneal carcinomatosis. Under the impression that the patient had ovarian cancer, the present surgical team carried out an explorative laparotomy and discovered the ruptured bilateral ovarian endometriomas. In this study, it is suggested that clinicians carrying out differential diagnosis of pelvic mass with high serum CA-125 level and ascites should consider not only ovarian cancer but also ruptured endometrioma.