After total thyroidectomy for medullary thyroid carcinoma, a 50-yr-old male was found to have decreased but still abnormally elevated plasma calcitonin levels. A thyroid scan with 131I revealed remaining thyroid tissue primarily in the form of a thyroglossal duct remnant. Two courses of 131I treatment of 30 and 150 mCi, respectively, produced a gradual decrease in plasma calcitonin. This study demonstrates that carefully selected patients may benefit from the use of 131I treatment as an adjunct to surgery in medullary thyroid carcinoma.