The National Research Council was asked by the Centers for Disease Control and Prevention (CDC) to review the draft report of the National Cancer Institute (NCI)-CDC's working group charged with revising the 1985 radioepidemiological tables. To this end, a subcommittee was formed consisting of members of the Council's Committee on an Assessment of the Centers for Disease Control and Prevention Radiation Programs and other experts. The original tables were mandated under Public Law 97-414 (the Orphan Drug Act) and were intended to provide a means of estimating the probability that a person who developed any of a series of radiation-related cancers, developed the cancer as a result of a specific radiation dose received before the onset of the cancer. The mandate included a provision for periodic updating of the tables. The motivation for the current revision reflects the availability of new data, especially on cancer incidence, and new methods of analysis, and the need for a more thorough treatment of uncertainty in the estimates than was attempted in the original tables.