appear move numerous in the expa@ed group, though no specific senctic. studies have becn carried out. aA persisting low levei of chromusomai aberrations of the type induced by radiation nave been noted £10 years post exposure. Frequent slit lamp observations reveci no increase in opacities of the lcns characteristic of radiation exposure. DEVELOPMENT OF THYROID ABNORMALITIES Before discussing development of Chyroid abnormalities a few words should be said concerning the calculations of the dose to the thyroid. Table II shows the estimates of the various isotopes thac were present in the people extrapolated back to day 1 based on radiochemical urine analyses. It can be noted thac the largest exposure was to the thyroid gland from radioiodines. In addition to the dose imparted by 13lz, the shortez-Lived isotopes 1327133], and 1351 also contributed substantially to the dose. Absorption of these iodine isotopes was both from inhalation as the radioactive cioud passed over and from drinking contaminated water and eating contaminated food. Based on radiochemical urine analyses beginning at 15 days after exposure it was estimated that about 11.2 _Ci of 131y was accumulated in the thyroid gland at the time of the fallout. The adult thyroid was estimated to have received about 160 rads from the radioicdines in addition to 175 rads from whole body gaama irradiation. The dose to the much smaller glands of. children was estimated to be considerably higher; in children less than 4 years of age the range was estimated to be 700-1400 rads. Annual examinations have aiways included careful. thyroid evaiuation. “1,2; Until 5 years ago the exposed people were considered to have normal thyroid function with no obvious evidence of any thyroid abnormality. Numerous serum protein bound iodine determinations had been done and ali appeared to be in the normal range for these people. We later discovered that Marshallese people in general have high levels of iodoprotcin which may have masked an earlier hypothyroid tendency.'3] Dietary and urinary iodine excretion were within the normal range. Serum cholesterol levels had been normal. Ten years after tallout exposure nodular changes were noted in a 12 year old exposed girl. Since that time increasing numbers of cases of nodularity of the thyroid gland have developed in the exposed peoole. At the present time there have been 21 cases of thyroid abnormalities, 19 with nodules and 2 other cases with atrophy of the gland with development of marked hypothyroidism. Only 1 of these cases with a nodular thyroid was in the lesser exposed Ailingnae group. Thirty-eight percent of the more heavily exposed group have displayed thyroid abnormalities. This includes one new case that was discovered in March 1969. A notable fact is that 90% of the children exposed at less than 10 years of age have developed such abnormalities. Table III shows the distribution: of thyroid cases in the populations under study. Of the 19 children in the 175 rad group exposed at less than 10 years of age, .only 2 have failed Soo 1b 14 -4e m—