The effect of 131I therapy as an adjunct to surgery in the management of patients with hereditary medullary carcinoma of the thyroid (MCT) was studied in 20 patients from 3 kindreds. Plasma calcitonin levels were measured before and after a dose of 131I sufficient to ablate postoperative thyroid remnants. In patients with residual biochemical MCT no significant reduction of the calcitonin levels was found after administration of radioiodine. In patients with normal postoperative calcitonin levels no (biochemical) recurrence developed 3 months to 2 yr subsequent to radioiodine therapy. It is concluded that 131I therapy as an adjunct to surgery is not indicated in the management of patients with residual hereditary MCT, although 131I may be of value in the prevention of tumor recurrence in patients with normal postoperative calcitonin values.