The Army and Navy medical missions which investigated the results of the explosion of the atomic bombs in Hiroshima and Nagasaki, Japan, studied many aspects of the situation. In this article it will be impossible to review all the studies that were made; only two major subjects will be considered. The two most impressive features of the new type of bomb are: (1) the tremendous toll of casualties and (2) the syndrome of injury due to large amounts of gamma radiation. In the bombed cities the Air Raid Defense authorities and the surviving members of the medical profession were confronted with an extremely large relief and rescue problem. The magnitude of their task is indicated by the following estimates (table 1): Of the total number of casualties, 120,000 in Hiroshima and 65,000 in Nagasaki, not all were in need of immediate medical care. In each city about one sixth of