More than 14 years have elapsed since Hertz1and Hamilton2first used radioiodine to treat thyroid disease. During this time reports3have been published on the treatment of more than 1700 hyperthyroid patients with radioactive iodine, I130and I131. Yet many questions remain unanswered. Does radioiodine, I131, cause cancer of the thyroid gland 10 to 20 years after a dose of I131 is given for the treatment hyperthyroidism?4Should one aim to "cure" the hyperthyroidism with one dose, or should multiple small doses be used to avoid a high incidence of hypothyroidism? Do severely hyperthyroid persons with an unusually high I131uptake require a smaller or larger dose of radioiodine than the average patient?5Should radioiodine be used as preoperative preparation for toxic nodular goiters?* Will a substernal goiter disappear by the time I131therapy has rid the patient of thyrotoxicosis? Is the I131method of relieving