More than 200,000 U.S. military personnel participated in atmospheric nuclear weapons tests between 1945 and the 1963 Limited Nuclear Test Ban Treaty. Questions persist, such as whether that test participation is associated with the timing and causes of death among those individuals. This is the report of a mortality study of the approximately 70,000 soldiers, sailors, and airmen who participated in at least one of five selected U.S. nuclear weapons test series1 in the 1950s and nearly 65,000 comparable nonparticipants, the referents. The investigation described in this report, based on more than 5 million person-years of mortality follow-up, represents one of the largest cohort studies of military veterans ever conducted.