with non-radioactive iodine; this should rarely produce anyserious side effects and would beof particular value in children and pregnant women. When exposure of the thyroid has already occurred, prophylactic treatment with thyroid hormone, now being used in the Marshallese, may help prevent development of thyroid tumors, and even after tumor development,surgical excision may reduce mortality due to malignancy. The thyroid effects in the Marshallese were not anticipated at the timeofthe accident or during the early years afterwards. In retrospectthis is not surprising, for several reasons. At that time the thyroid was thoughtto be relatively radioresistant, particularly with regard to radioiodine exposure (on the basis of animal studies and diagnostic and therapeutic use of 151] in peaple), and the calcu- lated thyroid doses in the Marshallese were considered to be below thelevels likely to produce _ tumors. In addition, neither the importanceof the exposureto short-lived iodine isotopesin fallout nor the thyroid dose differential in children due to the smallersize of their gland was fully appreciated. It is quite likely that the final results of thyroid lesions in the Marshallese are incompleteat this time since newlesions are still occurring. The been seen as late as 40 years after exposure. Fur- thermore, on the basis of the present data the risk of developing radiation-induced thyroid neoplasia is probably underestimated, since surgical removal of potentially malignanttissues may have occurred and the hormone treatment may haveinhibited the development of some tumors, althoughthelatter is questionable. As has been pointed out, also, the true carcinogenic potential of the exposure, particularly in the children receiving the higher dose, may have been masked by excessive cellular destruction. The recent finding that subclinical thyroid deficiency is present in some of the exposed people who have not shown any thyroid abnormalities indicates that the thyroid effects in the Marshallese may not yet be completely manifest, and continuedcareful surveillance of this population is necessary. Acknowledgments Because of the broad scope of the surveys and the complexity of operations it would be impossible to acknowledge all the individuals and organizations that have contributed to the success of the surveys over the past 20 years. We wouldlike, however, to express thanks to some whohave contributed vitally in more recent years. Ourforemost appreciation is to the people of Rongelap and U-irik who have participated in the examinations, including the unexposed people comprising the control population. Wetreasure the friendship andtrust of these people. We are grateful to many Rongelap and Utirik peopie for assisting the examining teams over the years, in- cluding the Magistrates of both islands - at Ron- gelap: John Anjain,Billiet Edmond, Jabwe Jorju, and Niktimos Antak; at Utirik: Kabto Campus, Maja Lena, Harold, the school principal, and Joata, the health aide. Thesenior author is most grateful to the many fine medical and otherspecialists (listed in Appendix 2) who haveso willingly given of their time andtalents, without remuneration,to participate in the surveys. Without their dedicated efforts the surveys could never have succeeded. Heis also greatly indebted to the many technical assistants who have contributed muchto the success of the surveys, particularly to Mr. W. Scott and Mr. D. ee a radioiodines by isotope dilution and saturation mean latent period for radiation-induced thyroid tumors may be as long as 30 years. Cases have _ posure under extreme conditions, with no corrective measures taken to reduceinternal absorption of radioisotopes prior to evacuation of the exposed people from thefallout area. {t did not involve the contaminated pasture-cattle-milk cycle, which might be an important pathwayof radioiodine to man in other types of accidents (such as the Windscale accident).1"6 Civil Defense planning can provide for several measures that will reduce the hazard of thyroid exposure dueto radioiodine absorption and thus largely preclude the degree of thyroid injury sustained by the Marshallese. Since the hazard from radioiodine is acute for a period only of days, early protective measures are extremely important. These include avoiding inhalation of radioiodines by shelter protection; consuming food and water only from closed containers; feeding cows in contaminated areas protected fodder; and temporarily withholding contaminated milk supplies and diverting them into processed products with a shelflife longer than thelife of the isotope. The addition ofstable iodine to food or water during the first week woulld provide a relatively inexpensive method of reducing thyroid uptake of 77 we ee re ees ee 75