102 LARSENetal. only two subjects, both about one year of age at exposure, have been found to have primary hypothyroidism. The recent availability of sophisticated immunoassay techniques for thyroxine (T,) and thyrotropin (TSH) has allowed more thorough thyroid evaluation of the exposed population who do not have known thyroid abnormalities (43 Rongelap people). Initially, prophylactic T, was discontinued for two months in a sample group of exposed subjects and 10 U of bovine TSH were given intramuscularly. Plasma T, was measured before and 24 hours after TSH. The mean incrementin T, was 2.4 + 1.2 pg/dl (mean + SD) in the exposed group, significantly less than the value of 4.2 + 1.3 yg/dl in controls. This suggested a decrease in thyroid reserve in exposed subjects. Accordingly, prophylactic T, treatment was discontinued for two months, and basal plasma T, and TSH, as well as the increment in TSH after Thyrotropin Releasing Hormone (TRH) was measured. The upperlimit of the normal basal plasma TSH was 3 pU/ml and of the TRH-induced TSH response was 22 U/ml in control Marshallese subjects. Four of 43 Rongelapese had abnormally high basal TSH and TRH-induced TSH release on two such tests as opposed to only two of 214 controls. Plasma Ts, concentrations were low, or low-normal in these individuals. These results indicate the presence of early thyroid dysfunction. Several other subjects have shown atleast one abnormal finding but have not had the required numberoftests to meet the established criteria. In three-quarters of these subjects the estimated thyroid exposure dose was < 400 rads. Hypothyroidism has been previously noted after therapeutic doses of "I for hyperthyroidism, but not in individuals exposed to therelatively low levels of thyroidal radiation (<400 rads) estimated for these individuals. BACKGROUND This report concernslate radiation effects on the thyroid in a population in the Marshall Islands inadvertently exposed to fall-out. The accident occurred on 1 March 1954, during the United States atomic testing programme when an unexpected shift of winds, following detonation of a thermonuclear device at Bikini, caused radioactive fall-out to be deposited on several inhabited islands to the east. Evacuation of exposed persons was accomplished by two days. The following were estimated whole-body gammadosesin the Marshallese on three atolls: Rongelap (64 people), 175 rads; Ailingnae (18 people), 69 rads; and Utirik (158 people), 14 rads. (There were also 28 American servicemen on the island of Rongerik who received about the same exposure as the Ailingnae group.) Acute effects of gamma exposure were noted in the Rongelap and Ailingnae groups, but not in the Utirik group. These consisted of early, transient anorexia, nausea and vomiting in a numberof people followed by depression of white blood cells and platelets to about half normal levels. Fortunately the haematological depression was not great enough to result in detectable clinical signs of infection or bleeding. No specific therapy was necessary and no deaths occurred, and blood cells returned to near normal levels by one year. In addition, in the Rongelap and Ailingnae groups, beginning about two weeks post exposure, radiation burns (‘beta’ burns) and spotty epilation of the head developed where fall-out material had been deposited on the skin. These burns were largely