exposure. The number of persons in the Rongelap group is small and many were children at the time of exposure, thereby introducing sample size and age factor into the analysis. Finally, variations in the thyroid-absorbed radiation dose were primarily dependent on ageat exposure, and therefore extensive destruction of thyroid tissue with a consequent decrease in risk of thyroid cancer may have occurred in the youngerindividuals (NCRP, 1985). One inference is extractable from the Marshallese data by examining the ratios of benign to malignant nodules (Table 3, column B). The Utirik population, which received no thyroxine suppression, had ratios of about 3.5:1 in children less than age ten and 3.7:1 in older children and adults, respectively. In the exposed Utirik group over the age of 10 the number of carcinomas which developed was 3 and the number of benign nodules was 11. On the other hand, in the Rongelap group over the age of10, which was receiving thyroxine suppression, the number of carcinomas that developed was 4 and the numberof benign nodules only 5 with a ratio of 1.25:1. Based on the numberofthyroid cancers in the Utirik group, the number of benign Rongelap nodules in those exposed when they were over 10 years of age should have been about 15. That the relatively low number of benign nodules in this group was not the result of the higher radiation dose is seen in the plethora of benign nodules and the highest ratio of benign to malignant nodules in Rongelap children under ten years of age, all of whom received over 2000 cGy. Thyroxine suppression mayhave resulted in the development of fewer benign nodulesin the older population. (See above for the discussion concerning the limitations of the validity of this interpretation). It is not possible to determineif thyroxine prevented the development of benign nodules in Rongelap children under 10 years of age, in part because 15 of the total of 18 adenomatous nodules in this group had been detected within five years of starting suppression therapy and therefore were unlikely to have been much affected by prophylaxis. The incidence of thyroid cancer in Rongelap persons over 10 years of age was 7% and in the comparable Utirik population persons 4%. The incidence in the former might have even been higher without thyroxine suppression but this will never be proven. 9) Did fetal radiation exposure produce thyroid nodules? It is known that 11 given in pregnancy can produce hypothyroidism in the fetus (Fisher et al., 1963). Since the fetal thyroid begins to concentrate iodine at about the twelfth week of pregnancy, risk of fetal thyroid injury from radioiodines begins at this time. Those fetuses at the time of exposure to BRAVOfallout received both a whole-body dose of gamma radiation equal to their mothers’ and a radioiodine dose to the thyroid which was a function of age of gestation, maternal radioiodine dose, and the extent of placental transfer of the radioisotope. The placenta is not a barrier to iodine transfer (Fisher, 1975). Twelve persons followed by the Marshall Islands Medical Program were in utero at the time of exposure, four from Rongelap and eight from Utirik. Three of these have now developed thyroid nodules: two of the four Rongelap children and one of the seven Utirik children (the eighth person in the latter group has never been available for examination). Table 5 summarizes the in utero exposure data. The finding that nodules (all benign) have occurred in at least 27 percent of those in utero at exposure is striking, particularly since the thyroid doses were not calculated to be very high in two of the three (Nos. 3 and 8, Table 5). None of those irradiated in utero have become spontaneously hypothyroid. Since the external whole-body dose estimates are probably fairly accurate, it may be that the internalized dose was higher in the three persons with nodules than was estimated. All the external and internal thyroid doses calculated by Lessard et al. (1985) were derived from a variety of data on radiation sources and conditions, and that report states that the maximum thyroid-absorbed dose could have been as much as four times the mean values used herein. Alternatively, the fetus may be more susceptible than the adult to radiogenic thyroid nodules and perhaps even more so than the juvenile thyroid. Among 2,802 Japanese atomic bombsurvivors who were in utero at the time of exposure, 16 have developed cancers that appearedafter the age of 14 years. One of these was a thyroid cancer which occurred in a person whose gestational age was 22 weeks at exposure (Yoshimoto etal., 1988). 24