A 75-year-old woman presented with the left ovarian mass, ascites, unilateral pleural effusion, and elevated CA125 levels. Clinical and radiological findings were highly suspicious for a malignancy. The patient underwent surgical excision of the uterus and infracolic omentum, and an intraoperative consultation was performed. Ovarian mass was diagnosed as struma ovarii and no other surgical procedure was performed. Histopatologic examination showed struma ovarii on the left ovary and bland-looking thyroid tissue implants on omentum and hernia sac. The patient diagnosed as highly differentiated follicular carcinoma arising from struma ovarii (HDFCO). No additional therapy was recommended to the patient and after a 53 months of follow-up, no recurrence was identified. HDFCO previously named strumosis is characterized by the presence of thyroid tissue in the peritoneal cavity with a bland morphology. Pseudo-Meigs syndrome with elevated serum CA125 levels in a 75-year-old lady is highly suspicious for malignancy, but HDFCO is an entity with a benign clinical course that shows the importance of intraoperative consultation. We describe the first case of HDFCO (strumosis) presenting as Pseudo-Meigs syndrome with elevated serum CA125 levels.Keywords: CA125, pseudo-meigs syndrome, struma ovarii, strumosis.