The Biodosimetry Assessment Tool (BAT) is a computer program developed by the Armed Forces Radiobiology Research Institute (AFRRI) as an aid to provide early diagnostic information (clinical signs and symptoms, physical dosimetry, etc.) to health care providers responsible for the initial management of radiation casualties after a major radiation incident. It is designed primarily to permit collection, integration, and archiving of data obtained from patients accidentally exposed to ionizing radiation. BAT also provides an early estimate of radiation dose using the timing of prodromal symptoms, clinical data, and various aspects of the time-dependent hematology profile. Collection of relevant data is facilitated by use of structured templates and user-friendly software. The BAT software is illustrated here using five historical cases from the U.S. Radiation Accident Registry maintained by the Radiation Emergency Assistance Center/Training Site (REAC/TS) for the U.S. Department of Energy.