OBJECTIVE To report an unusual case of persistent thyrotoxicosis after treatment of Graves' disease, because of coexistence of struma ovarii. METHODS We report the clinical history, imaging studies, laboratory and pathologic data, and treatment in a patient with persistent hyperthyroidism after surgical treatment of Graves' disease. In addition, we discuss some aspects of the pathogenesis of hyperthyroidism due to functioning struma ovarii. RESULTS A 42-year-old woman underwent near-total thyroidectomy for treatment of Graves' disease. Post-operatively, hyperthyroidism was still present. Methimazole was administered again, and performance of a 131I whole-body scan demonstrated a focus of intense uptake in the pelvis. Pelvic ultrasonography revealed a mass (11 by 8 by 7.1 cm) arising from the right ovary, with both solid and cystic components. Abdominal surgical exploration was performed, and the final histologic diagnosis was struma ovarii. The symptoms of hyperthyroidism diminished, and 3 weeks postoperatively, the thyroid hormone levels were in the hypothyroid range. CONCLUSIONS In patients with refractory hyperthyroidism after thyroid surgical treatment, radioiodine scanning should be performed to diagnose or exclude the functioning profile of ovarian masses.