Two thyroidectomized patients with a history of differentiated thyroid carcinoma are presented who had nonmetastatic mediastinal 131I uptake following therapeutic doses of 131I. Chest CT scans in both patients demonstrated an anterior mediastinal mass. Surgical excision in one patient and a percutaneous CT-guided fine needle aspiration biopsy in the other disclosed normal thymus tissue. Iodine-131 uptake in the anterior mediastinum in patients thyroidectomized for follicular or papillary thyroid carcinoma may represent the thymus.