74 myelogenous form, since his past hemograms showeda fairly consistent depression of neutrophil counts compared with those of other exposed boys of the sameage. The inability to demonstrate clear-cut aging effects in the exposed groupin spite of repeatedattempts with a variety of tests is in accord with the generally negative results of similar attempts in the much larger exposed Japanese populations. The absence of radiation-induced cataractsis not unexpected since the dose to the lens was probably below threshold and neutrons (known to have a higher RBE for cataract induction than gammaradiation) were not involved in the Marshallese exposure. E. THYROID EFFECTS The high incidence of thyroid neoplasms(in 27 of 86 exposed Rongelap people, including 3 with carcinomas) and the development in somechildren of hypothyroidism and growth retardation provide clear evidence for the seriousness of thyroid injury due to radioiodine absorbed from fallout associated with atomic detonations. Had notcareful medical studies in the population been instituted soon after the exposure and continued to the present date, the extent of thyroid effects might not have been discovered. Thyroid exposureis likely to be greater when individuals are exposed within 100 to 150 miles of the bomb if detonated near the ground. In such situations, if lethal exposure to penetrating radiation does not occur, the principal hazard appears to be the late developmentof thyroid tumors and leukemia. Since thelatter is ultimately fatal, it is more serious than thyroid malignancies, most of which are well differentiated and have an excellent prognosis. The relative incidence of thyroid malignancies from radiation appears to be about the sameas that of leukemia on a “per rad” basis.177 The incidence of thyroid cancer is considerably higher than the mortality from it. Of 40 cases among the exposed Japanese, 34 wereliving in 1973, and only one death had been attributed to this cause. No evidence of thyroid dysfunction had been noted at the time of diagnosis. In the Marshallese reduced function was not found in two of the cancer cases exposed as adults, but was in the case exposed as a child. The high incidence of thyroid effects in children exposed at < 10 years of age was no doubtrelated to the higher dose to the child’s thyroid because of its smaller size. The rapid growth of the gtands during childhood probably increased the chances for neoplastic changes. The growth retardation in someof these children was thought to be related to reduced thyroid function resulting in lower hor- monelevels. This deficiency was not recognized during the early vears because offalsely high PBI levels resulting from unusually high iodoprotein levels, which turned out to occur generaily in the Marshallese and are now being further studied. Mostof the thyroid glands of the exposed people undergoing surgery contained multiple nodules or areas of adenomatous change. Many microscopic areas, although considered benign, were composed of discrete areas of atypical cells, suggestive in somecases of malignantpotential. Therisk of developing benign and malignant neoplasms in the Rongelap people appeared to be about the sameas that noted in people exposed to x-radiation. Clinical experience with 131] suggested that the risk would have been less, but the higher energy of the short-lived isotopes of iodine (particularly 132], 133], and '351), resulting in higher dose rate and more uniform exposure ot the thyroid, is thought to have been the important factor in increasing the numberofthyroid abnormalities above that expected from similar doses from 131J alone.It is not unreasonableto speculate that tumor incidence in the Marshallese would have been considerably smaller if only 1311 had been involved in the exposure. The lesser amount of short-lived iodine isotopes in the Utirik expo- sures (because of the later arrival of the fallout) may have been an important factor in reducing the doseeffect to their thyroids, but the number of people involved is too small for any firm conclusion to be drawn. Treatmentof the exposed Rongelap people with thyroid hormonehas been of benefit in enhancing growth and developmentin the growth-retarded children and in maintaining a normal metabolic state in the operatedcases.It is not certain whether it has prevented the development of thyroid nodules. The documentation of these thyroid effects has importancenot only for the people involved but also for the advancementof medical knowledge and for planning with regard to Civil Defense and remotely possible future accidents involving release of radioiodines(e.g., from a nuclear power plant). The Marshallese accident represents ex-