Summary Persons who were present on March 1, 1954, at Rongelap Island, Rongelap Atoll, Sifo Island, Ailingnae Atoll, and Utirik Island, Utirik Atoll in the Marshall Islands have been examined by medical specialists to determine if any observable effects occurred as a result of exposure to radioactive fallout from the Pacific weapon test known as Operation Castle BRAVO. Medical specialists have reported short-term effects exhibited over a period of many months and possible long-term effects exhibited over many years. A study was undertaken to reexamine thyroid-absorbed dose estimates for people who were exposed accidentally at Rongelap, Sifo, and Utirik Islands. The study included: 1) reevaluation of radiochemical analysis to relate results from pooled urine to intake, retention, and excretion functions, 2) analysis of neutron irradiation studies of archival soil samples to estimate areal activities of the Lodine isotopes, 3) analysis of source term, weather data, and meteorology functions predicting atmospheric diffusion and fallout deposition to estimate airborne concentrations of the iodine isotopes, and 4) reevaluation of radioactive fallout contaminating a Japanese fishing vessel in the vicinity of Rongelap Island on March 1, 1954, to determine fallout components. The relative location of the exposed people is given as Figure Il. The original estimates of external whole-body dose from the acute exposure were 1.75 gray (175 rad) at Rongelap and 0.14 gray (14 rad) at Utirik (Cr56). The first estimate of thyroid dose from internal emitters in Rongelap people was 100 to 150 rep (Cr56). Thus the first estimate of total thyroid absorbed dose was 2.68 to 3.15 gray (268 to 315 rad) for Rongelap people in general and for internal plus external exposure. In 1964, three teenage girls who were exposed in 1954 underwent surgery for benign thyroid nodules. In 1964, 3 to 4-year-old child thyroid dose was reexamined by James on the basis 9f 1) urine bioassy results and 2) a range of values for thyroid burden of I, thyroid mass, and uptake retention functions for iodine (Ja64). In addition two modes of intake were considered, inhalation and ingestion. For 3 to 4-year-old girls the extreme range of thyroid dose from internal emitters was estimated at 2 to 33 gray (200-3300 rad). The most probable total thyroid dose was in the range of 7 to 14 gray (700-1400 rad). The James estimate of most probable total thyroid absorbed dose to the child was 2 to 5 times higher than the estimate reported by Cronkite for Rongelap people. The value for the James estiamte of total thyroid dose was extrapolated to other ages and to the Utirik people and reported along with medical effects by Conard (Co74),. The number of radiation-induced thyroid lesions per million person-rad-years at risk was tabulated by Conard for the Rongelap and Utirik exposed populations. It was clear that the risks of radiation induced benign and cancerous lesions were not comparable between the two atolls for any age grouping. The thyroid cancer risk for the Japanese population exposed at Nagasaki and Hiroshima, as reported by the National Research Council's Committee on the Biological Effects of Ionizing Radiation, was 1.89 excess cases per million person-rad~years of tissue dose (CBEIR80). This parameter was 7.0 at Rongelap and 17.8 at Utirik for the 10-year and older age grouping in 1974 (Co74),