A patient with well-differentiated thyroid carcinoma had a whole body scan using 5 mCi 131I which demonstrated abnormal uptake of 131I in a palpable pelvic mass. Approximately 24 years ago the patient had a total thyroidectomy followed by 131I treatment. The histologic examination of the mass was consistent with serous cystadenoma of the ovary. No thyroid tissue or teratoma was identified. The mechanism for the unusual uptake of 131I in a benign ovarian tumor is not clear, and the differential diagnosis of metastatic thyroid cancer is briefly reviewed.