INTRODUCTION Subsequent to World War II, the United States carried out several series of atmospheric tests of nuclear weapons in the Northern Marshall Islands between the years 1946 to 1958. Om March 1, 1954, at Bikini Atoll, BRAVO, the first of six nuclear weapons tests in the Castle series, was detonated. Due to an unanticipated wind shift, the BRAVO device produced substantial surface contamination on inhabited atolls up to 500 kilometers east of Bikini within a 5,000 square kilometer area. The contaminated region was cucumber shaped and falling bomb debris was visible on Rongelap Atoll from 5 to 10 hours after detonation (G162, Sh57). Following a fallout alert by a Navy monitoring team stationed at nearby Rongerik Atoll, the 64 residents of Rongelap Atoll and an additional 18 Rongelapese who were gathering food nearby at Sifo Island, Ailinginae Atoll, were removed to Kwajalein Atoll, some 300 kilometers to the south on March 3, 1954. On March 3 and 4, removal of the more distant 157 Utirik Atoll residents was affected. During the first few weeks and at least once every year from 1957 to the present, a Brookhaven National Laboratory medical team, organized by the Atomic Energy Commission (and its successor organizations) and the Department of Defense has regularly conducted medical examinations to monitor the health and to evaluate the radiobiological status of persons affected by tropospheric fallout from the BRAVO nuclear test. Reports of their findings including whole-body counting data and urine activity concentration data are available in Cr56, Du56, Du57, Wo59, Co56, 58, 59, 60, 62, 63, 65, 67, 70, 75, and Co80a. These reports may be consulted in order to easily follow the information presented here. Estimates of the initial body burdens of internal emitters were presented in Co55, Coh56 and Coh60 and will