I. INTRODUCTION 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 can be attributed to exposure to radioactive fallout. Their 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. Medical specialists have reported short-term effects exhibited over a period of many months and possible long-term effects exhibited over many years. In 1964, three teenage females who were exposed in 1954 underwent surgery for benign thyroid nodules. In 196%, 3- to 4-year-old child thyroid dose was reexamined by James on the basis of 1) urine bioassay results and 2) a range of values for thyroid burden of 131y, thyroid mass, uptake retention functions, and ingestion or inhalation. 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 two to five times higher than the estimate reported by Cronkite for Rongelap people. The value for the James estimate 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 (Co74). It was clear that the risks of radiation-induced benign and cancerous lesions for the two atolls were not comparable for any age grouping. The thyroid cancer risk for the Japanese population exposed at Nagasaki and Hiroshima, in units 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 tO-year and older age grouping in 1974 (Co74). Variation in risk of radiation-induced thyroid cancer between atolls and the difference when compared to other irradiated groups had become an important scientific and health-related question with considerable political overtones. Early in 1977, Bond, Borg, Conard, Cronkite, Greenhouse, Naidu, and Meinhold, all members of Brookhaven National Laboratory (BNL), and Sondhaus, University of California, College of Medicine, initiated a reexamination of the technical issues. In 1978, formal program objectives and funding were supplied to BNL by the Department of Energy's Division of Biological and Environmental Research. *an obsolete unit of absorbed dose; 1 rep = 0.93 rad for soft tissue.