ABSTRACT Since Marth 1954, persons who were present at Rongelap Island, Rongelap Atoll, Sifo Island, Ailinginae 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. Studies indicated short-term effects exhibited over a period of many months and possible long-term effects exhibited over many years. In order to estimate the risk of possible ra- diation induced thyroid effects, a study was undertaken to refine thyroid absorbed dose estimates for people at Rongelap, Sifo and Utirik Islands using four approaches: (1) relating radiochemical analysis data on March 1954 pooled urine to current intake, retention and excretion models in order to determine I-13] inhaled or ingested, (2) estimating airborne concentrations and areal activities of the iodine isotopes from neutron irradiation studies on archival soil samples, (3) airborne concentrations and areal activities of the iodine isotopes derived from source term, weather data and current computer models which predicted atmospheric diffusion and fallout deposition and (4) determining fallout components based on Bikini Ash, the radioactive fallout which fell on a Japanese fishing vessel in the vicinity of Rongelap Island. Bikini Ash was also used to derive air, water and surface activities of fallout nuclides. This re-examination resulted in a greater mean thyroid absorbed dose estimate (by a factor of 4) over that estimated by James (Ja64). A wide range of possible thy- roid absorbed dose due to a wide range of activity intake was also a result. Direct inhalation and ingestion of contaminated water were pathways which contributed in a minor way to fallout activity intake. The pathway contributing to most of the intake was fallout debris falling directly on food prepared and