nuclear weapons [1]; it is located south of the City of Ekaterinburg and north of the City of Chelyabinsk ( Fig. 1). Construction of the MPA facility began in 1945 and start-up of the plant occurred in June 1948 with the operation of the initial uranium-graphite reactor for the production of plutonium. The first radiochemical plant was placed on-line in December 1948, and the production of enough plutonium for an initial weapons-related test soon followed. Over the intervening years a total of seven reactors have operated at this site; five uranium-graphite reactors have now been decommissioned. The two remaining reactors are operated mainly for the production of isotopes. Since 1977 the radiochemical plant has been used extensively to reprocess fuels from power reactors and from transport and research reactors. The radioisotope plant dates back to 1962 and is now one of the major suppliers of sources and preparations of radionuclides.The early years of operation of the MPA are of most interest from the standpoint of radiation exposure of the workers and the nearby general population. The entire enterprise consisted of an industrial zone of 90 km 2 surrounded by a buffer zone of 250 km 2 . Throughout the operation of the MPA, its workers have lived in the closed City of Ozyorsk, which is located several kilometres west of the MPA. During 1948 and 1949, as the MPA became a fully operational plutonium manufacturing site, the enterprise consisted of several facilities located at a distance of 500 m-5 km from the main site. These facilities consisted of q Reactors (group F plants) q Radiochemical plants (group B plants), including waste treatment and high-level waste storage (complex C) q Industrial complex for production of 239 Pu (group V plants)There was great pressure during the early years of operation of the MPA to produce enough plutonium for the initial bomb test and to produce additional weaponsgrade material. Due to this urgency and to the lack of proper equipment and technological controls, substantial exposure occurred to both the workers at the enterprise and to the general population living in the City of Ozyorsk and in villages downstream of the MPA on the Techa River (Fig. 1). These exposed populations form a unique resource for the study of radiation effects in humans. The worker cohort includes a high fraction of women, which is unusual. Several cohorts have been formed of members of the unselected general population; these cohorts are large and have relatively large exposures. The major exposures occurred approximately 50 years ago, and a substantial effort has been made to follow the more important cohorts and to determine their vital status. Thus, these populations are comparable in many ways to the survivors of the atomic bombings in Japan. That is, the populations are large, the follow-ups have been active, the exposures are large, members of both sexes have been exposed, the members of the general population cohorts were unselected, and the time of follow-up is already long. A major difference...