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The US Atomic Energy Commission carried out over 100 aboveground tests of nuclear weapons at its Nevada test site, which led to the deposition of radioactive fallout in sections of Utah and Nevada. Studies that have associated the fallout with higher leukemia rates in the affected counties have reported very different effects, ranging between a 1.5-fold increase and a 5.3-fold increase in leukemia. However, these reports estimated the amount of exposure based on the county of residence at the time of death, rather than on where the person lived when exposure was greatest. To improve accuracy, radiation doses received by people who had died with or of leukemia (for chronic lymphocytic leukemia, the patient must have died of the disease) in Utah were calculated based on residence information obtained from the church registry of the Church of Jesus Christ of Latter-day Saints, an organization that keeps a file of deceased members. Other residence and age criteria were established. Radiation doses were calculated for people who met these criteria (1,177 cases) and age- and sex-matched controls (5,330 people who met the residence criteria, but who died of causes other than leukemia). Dosages were calculated according to the average total dose to bone marrow received during the period of fallout, and regional differences within Utah are reported. While weak associations between the dose to bone marrow and leukemia at all ages and all times after exposure were found, they were not statistically significant. However, a positive association was found for people who died before reaching the age of 20 years and between 1952 to 1957. No association was found for people who had been in utero at the time of maximal exposure. An evaluation of alternative explanations to radiation to account for the results is presented. In summary, and with the consideration of findings from other studies, it is possible that the excess rate of acute leukemia in southwestern Utah is due to fallout. However, the number of such cases is small. (Consumer Summary produced by Reliance Medical Information, Inc.)