Given the wide usage and proven value of the Abbreviated Injury Scale (AIS) in rating severity of trauma, it is essential that certain reliability issues concerning its application be resolved. This article describes a study designed to address these reliability issues. Each of 15 raters with varying qualifications was asked to identify AIS code injuries sustained by 375 trauma patients admitted to four Baltimore area hospitals. Results showed that as a group, physicians and nurses tend to be more reliable in their ratings than either emergency medical technicians (EMTs) or nonclinical technicians, although a research assistant who is well trained in AIS coding and is a diligent worker can use the AIS to code severity as reliably as the physicians when sufficient information is provided in the medical chart. Reliability of AIS scoring was somewhat higher for blunt (vehicular and nonvehicular) versus penetrating injuries.