Ionizing radiation and its sources are used every day in medical, industrial and governmental facilities around the world. Although some health risks from ionizing radiation exposures are widely recognized, the association of these exposures to specific diseases, especially various types of cancer, remains uncertain. Workers at U.S. Department of Energy (DOE) facilities have produced nuclear weapons, provided nuclear fuel materials for power reactors, and conducted a wide spectrum of research related to nuclear safety and other scientific issues. While completing this work, many of the employees have been exposed to ionizing radiation and other potentially hazardous materials. Since 1991, the National Institute for Occupational Safety and Health (NIOSH) has conducted analytical epidemiologic studies of workers at DOE nuclear facilities, through a Memorandum of Understanding between the DOE and the U.S. Department ofHealth and Human Services (DHHS). The agreement occurred in response to recommendations to the Secretary of Energy in 1989 by the independent Secretarial Panel for the Evaluation of Epidemiologic Research Activities (SPEERA). This technical report, entitled An Epidemiologic Study of Mortality and Radiation Risk of Cancer Among Workers at the Idaho National Engineering and Environmental Laboratory, a U.S. Department of Energy Facility, is one several products of the NIOSH Occupational Energy Research Program that are being published as a series. Most of these studies include detailed historical exposure assessments for radiation and other potentially hazardous agents so the health risks at different levels of exposure can be accurately estimated. Each of these studies contributes to the knowledge required to ensure that workers are adequately protected from chronic disease over their working lifetimes. The Idaho National Engineering and Environmental Laboratory (INEEL) is a large U.S. Department of Energy (DOE) facility near Idaho Falls, Idaho. Since its construction in 1949 the INEEL has conducted a wide variety of activities, including engineering and basic scientific research, nuclear reactor design and testing, nuclear material chemical processing, and the construction, servicing and demolition of large-scale facilities. In addition, the U.S. Navy maintains its Naval Reactors Facility (NRF) at the INEEL, where research and testing of Navy ship reactors occurs, as well as training of military and civilian personnel involved in the naval nuclear surface ship and submarine program. An epidemiologic cohort mortality study was initiated to evaluate hazards associated with ionizing radiation and other exposures among civilian employees at the INEEL facility.