74 myelogenous form, since his past hemograms showed fairly consistent depression of neutrophil counts comparedwith those of other exposed boys of the same age. Theinability to demonstrate clear-cut agingeffects in the exposed groupin spite of repeated attempts with a variety oftests is in accord with the generally negative results of similar attempts in the much larger exposed Japanese populations. The absenceof radiation-inducedcataractsis 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 Mar- shallese exposure. E. THYROID EFFECTS The high incidence of thyroid neoplasms(in 27 of 86 exposed Rongelap people, including 3 with carcinomas) and the developmentin somechildren of hypothyroidism and growth retardation provideclear evidencefor 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 continuedtothepres- ent 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 moreserious 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 Theincidence of thyroid cancer is considerably higher than the mortality from it. Of 40 cases amongthe exposed Japanese, 34 wereliving in 1973, and only one death had beenattributed to this cause. No evidenceof thyroid dysfunction had been notedatthe timeof diagnosis. In the Marshallese reduced function was not found in two of the cancer cases exposed as adults, but wasin 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 glands during childhood probably increased the chances for neoplastic changes. The growth retardation in someofthese children was thoughtto be related to reduced thyroid function resulting in lower hormonelevels. This deficiency was not recognized during the early years because offalsely high PBI levels resulting from unusually high iodoprotein levels, which turned out to occur generally in the Marshallese and are now being furtherstudied. 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 malignant potential. The risk 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] sug- gested that the risk would have beenless, but the higher energy of the short-lived isotopes of iodine (particularly 1371, 1331, and 1351), resulting in higher dose rate and more uniform exposure ot the thyroid,is thought to have been the important factor in increasing the numberof thyroid abnor- malities above that expected from similar doses from 151f alone. It is not unreasonable to speculate that tumorincidence 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 exposures (because of the later arrival of the fallout) may have been an important factor in reducing the dose effect to their thyroids, but the numberof people involved is too small for any firm conclusion to be drawn. Treatmentof the exposed Rongelap people with thyroid hormone has 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 involvingrelease of radioiodines(e.g., from a nuclear power plant). The Marshallese accident represents ex-