A large superior mediastinal mass was found incidentally by ultrasonography in a 60-year-old man. There was an abnormal accumulation of Tl-201 in the lower pole of left thyroid gland, extending into left superior mediastinum on Tc-99m pertechnetate/Tl-201 subtraction scan. Laboratory findings relating thyroid and parathyroid were all within normal range. We considered the mass as a non-functioning parathyroid adenoma tentatively. However, subsequent surgery and pathologic examination revealed the mass to be a benign mixed thymoma. We report a case of patient with thymoma showing unusual Tc-99m pertechnetate/Tl-201 subtraction imaging and laboratory findings, and suggest to consider the possibility of other mediastinal tumors rather than parathyroid adenoma.