Their Possible Bearing on the Use of Radioactive Iodine To the Editor:— Radioactive iodine, especially the isotope of 8 day half-life, I131, has been employed in the treatment of hyperthyroidism in several clinics during recent years, a total of several hundred patients probably having received the material. The question of the possibility of late radiation changes leading to malignant degeneration has been raised and cannot be disregarded. Serious sequelae to radiation therapy usually appear several years after the treatment; hence such damage from radioiodine would not be expected for some time. If it is to be feared that this will be a serious hazard, the use of the material ought to be very sharply restricted until sufficient time has elapsed to ascertain the late results in persons already treated. On the other hand, if any pertinent evidence can be obtained from the already large experience with roentgen rays, it