Meigs' syndrome is the association of ovarian fibroma, pleural effusion, and ascites. Meigs' syndrome with marked elevation of CA125 is an unusual clinical condition reported in 27 cases in the literature. The patient was a 46-year-old woman with right pleural effusion, ascites, ovarian tumor, and CA125 level of 1808 U/mL. Tomography revealed ascites and bilobate pelvic tumor of approximately 25 cm. The diagnosis of advanced epithelial ovarian cancer was considered, and the patient was treated with chemotherapy. Three chemotherapy schemes were applied due to the total lack of response in tumor volume; however, CA125 decreased to 90 U/mL. Thus, surgery was performed with resection of 25 cm of the left ovarian tumor, with intact capsule and without implants; the result of histopathologic analysis was fibroma. Postoperative CA125 was 11 U/mL. Patients with elevated CA125 and ascites cytology positive for malignancy must be cautiously treated due to the possibility of false positives, even if the probability is low. Therefore, minimally invasive surgery for biopsy collection must be considered. Although the association between ovarian tumor, pleural effusion, ascites, and marked elevation of CA125 is highly indicative of epithelial ovarian cancer, Meigs' syndrome must be considered in the differential diagnosis.