Purpose: We designed a retrospective study to measure the accuracy of the ICD-10 (International Classification of Disease-10) code for trauma patients. We also analyzed the error of the ICISS (International Classification of Disease based Injury Severity Score) due to a missing or an incorrect ICD-10 code. Methods: For the measuring the accuracy of the ICD-10 code for trauma patients in a tertiary teaching hospital`s emergency department, two board certified emergency physician performed a retrospective chart review. The ICD-10 code was classified as a main code or a sub-code. The main code was defined as the code of the main department of treatment, and the sub-code was defined as a code other than the main code. We calculated and compared two ICISS for each patient one by using both the existing code and the other by using a corrected code. We compared the proportions of severe trauma (defined as an ICISS less than 0.9) between when the existing code and the corrected code was used respectively. Results: We reviewed the records of 4287 trauma patients who had been treated from July 2008 to November 2008. The accuracy of the main code, the sub-code of emergency department, main-code, the sub-code of hospitalized patients were 97.1%, 59.8%, 98.2% and 57.0%, respectively. Total accuracy of the main and sub-code of emergency department and of hospitalized patients were 91.4% and 58.6%. The number of severe trauma patients increased from 33 to 49 when the corrected code was used in emergency department and increased from 35 to 60 in hospitalized patients. Conclusion: The accuracy of the sub-code was lower than that of the main code. A missing or incorrect sub-code could cause an error in the ICISS and in the number of patients with severe trauma. (J Korean Soc Traumatol 2009;22:108-115)