THE TREATMENT of hyperthyroidism with radioiodine results from the intraglandular radiation therapy of the abnormal thyroid. This report is concerned with the first 100 consecutive patients to whom radioiodine was administered for the treatment of hyperthyroidism at the University of California Medical School since the present radioiodine laboratory was established in 1945. The patients whose data are presented came from the thyroid clinic of the University of California Medical School and from private physicians. They were accepted for radioiodine treatment of their hyperthyroidism when in the opinion of referring physicians and the physician in charge of the radioiodine laboratory (E. R. M.) they had hyperthyroidism; they were not pregnant; there were no palpable nodules in the thyroid; they were apparently responsible enough to return for follow-up evaluation; they lived near enough to San Francisco so that the follow-up visits were not an undue burden, and they were not under the