FALLOUT EFFECTS—-MARSHALL ISLANDERS 65 to uncertainty regarding amounts of contaminated milk consumption.) It is not knownif short-lived isotopes of iodine were involved. Thyroid examination of children from that area by thyroid experts 12 to 17 years later failed to reveal any increase in abnormalities of the gland (49). Calculation of risks (cases/10® persons/rad/year) for benign and malignant thyroid nodules, based on present estimates of the doses to the Marshallese, gave results in the same rangeas therisks reported for x radiation (3,6,30,56,60). Therisk calculated for thyroid hypofunction was considerably higher than that reported following I-131 therapy (4,18). In interpreting these findings one has to keep in mind the uncertainty in the dose to the Marshallese. Preliminary results of reevaluation of thyroid doses at this laboratory indicate that the present estimates maybe too low (39). If so, this would lowerthe risk estimates. However, one could not accountfor the findings on the basis of gammaor I-131 exposures alone. A large component of the thyroid dose in the Rongelap children was from radioiodines. The short-lived isotopes (I-132,I-133,I-135) are believed to have contributed two to three times the dose from I-131. A limited number of animal studies comparing the effectiveness of I-131 with that of x radiation (3,48,56,57) and of short-lived iodine isotopes (6,7,20,34,59,62) indicate that I-13] is less effective in producing thyroid abnormalities. The greater effectiveness of the short-lived isotopes seems to be related to more energetic beta rays and higher dose rate. It is unfortunate that more definitive studies of this type are not available, particularly with smaller doses of radioiodines. Even so, on the basis of available data, the possible importance of the short-lived radioiodines in the Marshallese exposure must be considered. It should be pointed out with regard to the risk associated with thyroid hypofunction that sensitive biochemical techniques, not generally used, were employed in the Marshallese. Also, the more severe degrees of hypofunction were noted in the children, whereas most risk estimates for hypothyroidism are based on overt cases almost exclusively in adults treated for hyperthyroidism. Doniach (19) has postulated a multistage development of radiation-induced thyroid neoplasia with radiation as the initiating factor and with secondary or promoting factors comprising mainly TSH stimulation secondary to glandular hypofunction and including increased growth rate and metabolism ofthe thyroid in children and possibly stresses of puberty and pregnancy. In addition to the increased doses in the children due to smaller glands, these promoting factors probably played a role. It was noted that most of the women who developed thyroid carcinoma had multiple pregnancies in the years prior to detection of their malignancy. Even though the thyroid dose estimates in the Marshallese are uncertain and the number of cases is small, there are certain dose-effect relationships that seem likely. In Fig. 1 the percent occurrence of thyroid abnormalities is plotted according to dose range for the different groups (regardless of age). The data for the six Rongelap children exposed at 1 year of age are plotted separately, since they probably received doses that could be as high as 2,000