I. A. INTRODUCTION Background In March 1954, an event as unprecedented as it was unfortunate occurred in the Central Pacific. Radioactive debris from a thermonuclear weapon test at Bikini Atoll deviated from predicted trajectories and contaminated several atolls in the northern Marshall Islands. As a result, 239 native inhabitants of these islands along with 28 American servicemen and 23 Japanese fishermen received variably severe exposures to diverse tonizing radiations. Fallout material consisted largely of mixed fission products with small amounts of neutron-induced radionuclides and minimal amounts of fissionable elements, producing a complex spectrum of electromagnetic and particulate radiation. Individuals were exposed to deeply penetrating, whole-body gamma irradiation, to internal radiation emitters assimilated either by inhalation or by ingestion of contaminated water and food, and to direct radiation from material accumulating on body surfaces. That tragic accident initiated a cascade of events, medical, social and political, which continue in varying forms to this day. Most of these have been discussed in the open medical literature and in periodic reports issued by the medical team headquartered at Brookhaven National Laboratory. This re- port attempts to summarize some of the principal findings of medical significance that have been observed during the subsequent 26 years with particular emphasis on the last six years. Because of the unique nature of the incident, medical care of the exposed Marshallese and observation for detection of potential radiation effects have been inextricably bound to complex sociological and political considerations, mentioned in the Preface. The body of the present report, however, will be confined to medical observations on the exposed Marshallese. A 25year summary of the findings in the Japanese fishermen is presented in Appen- dix VII. Following the 1954 examinations of the American servicemen, they were returned to the custody of the U.S. Army and have not been examined by the medical team since that time. Expanded presentations of certain aspects of these studies can be found in the 20-year report (1), which is a comprehenSive review, and in previous annual reports (2-13), as well as in open litera- ture reports (14-39). B. The Accident In the early morning of 1 March 1954, a thermonuclear device code-named Bravo was detonated on a tower at Bikini Atoll as part of the Operation Castle nuclear weapons test series. The energy yield of this experimental device exceeded predictions, and sudden wind structure alterations sent the resultant cloud of radioactive debris unexpectedly eastward rather than over open seas to the north. Marshallese inhabiting Rongelap, Ailingnae, and Utirik Atolls and a group of American servicemen on Rongerik Atoll were caught within the downwind fallout field for two to three days before they could be evacuated to , Kwajalein Atoll by Navy units. In addition, the Japanese fishing vessel,