Introduction On Marouw 1, 1954, an experimental thermonuclear device was exploded at the U.S, Atomic Energy Commission’s Eniwetok Proving Grounds in the Marshall Islands. Following the detonation, unexpected changes in the wind structure deposited radioactive materials on inhabited atolls and on ships of Joint Task Force #7, which was conducting the tests. Radiation surveys of the areas revealed injurious radiation levels; therefore, evacuation was ordered, and was carried out as quickly as possible with the facilities available to the Task Force. Although the calculated accumulated doses to the exposed human beings were believed to be well below levels that would produce serious injury or any mortality, the Commander of the Task Force requested the Department of Defense and the U. S. Atomic Energy Commission to organize a medical team to provide the best possible care of the exposed persons and to make a medical study of the exposures. Responsibility for organization of the medical team was shared by the Armed Forces Special Weapons Project, Department of Defense, and the Division of Biology and Medicine, U.S. Atomic Energy Commission. Experienced professional and technical personnel were immediately available from the Naval Medical Research Institute and the U. S. Naval Radiological Defense Laboratory. Since speed was essential in the organization and transport of the medical team to the mid-Pacific area, the assistance of the Medical Departmentof the Navy was requested, and was promptly received from the Surgeon General. A team was organized from personnel of the two Navylaboratories and representatives of the AEC Division of Biology and Medicine and the Armed Forces Special Weapons Project. The team was air lifted to the Marshall Islands, arriving on the eighth day after the explosion. Interim care and study had been capably handled by the small medical departmentof the U. S. Naval Station, Kwajalein, Marshall Islands The commanderof the naval station had arranged living facilities for the exposed Marshallese, and installed laboratory and clinical facilities as requested immediately upon arrival of the medical team. Full cooperation and support from all agen- cies in the field enabled the medical team to operate at maximumefficiency, so that the de- gree of radiation injury could be assessed quickly, and appropriate care and study of the injured could be instituted without delay. All of the exposed individuals have recovered from the immediate effects without serious sequelae. Nevertheless it is planned to evaluate the medical and genetic status of the group at appropriate intervals with a view to learning what if any of the knownlate effects of radia- tion exposure may be observed. Obviously and indeed fortunately the number of persons re- ceiving 75 roentgens exposure and greater is too small to make it possible to determine with any degree of accuracy the effect on life span. In addition to providing medical care for these persons, the team accumulated a large body of scientific observations on radiation injury in human beings. The initial data have been supplemented by field resurveys 6, and 24 months after the original investigation. The results of this work are summarized in the present volume. The data which were ob- tained substantially increase the fundamental knowledge of radiation injury and the medical capability of caring for persons exposed to large doses of radiation. Cuaries L. Dunya, M. D., Director, Division of Biology and Medicine, U. S. Atomic Energy Commission. Im