A review and analysis of the dose response relationship for the probability of acute lethality from prompt or short-term exposure to ionizing radiation is presented. The purpose of this analysis is to provide recommendations concerning estimates of casualties expected from radiation accidents, the use of nuclear weapons, or possible terrorist activities. Previous work on acute ionizing radiation-induced lethality risk together with a collection of dose response relationships are analyzed and presented based on historical case data and expert opinion that have evolved from whole-body radiation therapy experience, radiation exposure accidents, nuclear weapon detonations, and animal experimentation. The nature of the data reviewed ranges from direct individual events to those offered according to collective expert opinion and consensus published as journal articles and in various technical documents and reports. The dose response relationships are expressed as two-parameter (median exposure level and slope) probability distribution models as a function of radiation exposure in terms of a free-in-air dose. Twelve different dose response relationships are presented and discussed, including the impact of some medical care.