Our goal was to adapt current diagnostic methods for radiation overexposure patients into a practical system that can be implemented rapidly and reliably by responders unfamiliar with the effects of radiation. Our Radiation Injury Severity Classification (RISC) system uses clinical and haematological parameters from the prodromal phase of the acute radiation syndrome (ARS) to classify acute radiation injury for purposes of managing treatment disposition. Data from well-documented ARS cases were used to test the RISC system. Three-day summaries were generated for each case. These were individually reviewed by the three physicians most involved with the development of the system to establish both a consensus case score (CCS) and disposition category ranges. 30 volunteer raters from varying health disciplines using the RISC system then each independently rated a random selection of 12 cases for injury severity in a self-trained field-simulation exercise. The CCS identified discrete cut-off ranges for the three disposition categories in both manageable and mass casualty events. The group of raters, after a modest period of self-training, achieved overall levels of pairwise agreement with the CCS category of 0.944 for manageable events and 0.947 for mass casualty situations. In conclusion, an early assessment of the severity of the ARS injury is required for an appropriate disposition determination. The RISC system should produce reasonably accurate and reliable assessments of radiation injury severity within 6-12 hours post exposure despite the probable absence of physical dosimetric data.