Background. Struma ovarii is a rare form of ovarian neoplasm composed entirely and predominantly of thyroid tissue. The association of pseudo-Meigs syndrome, elevation of CA125 and hyperthyroidism to struma ovarii is a rare condition. Case. We report an unusual presentation of a postmenopausal woman with benign struma ovarii associated with pseudo-Meigs syndrome, hypertiroidism, and elevated CA125 serum level, and a large complex right pelvic mass thereby mimicking an ovarian cancer. Conclusions. Struma ovarii is a rare cause of ascites, hydrothorax, elevated CA125, and hyperthyroidism. This rare condition should be considered in the differential diagnosis in patients with ascites and pleural effusions but with negative cytology. (c) 2004 Elsevier Inc. All rights reserved.