The purpose of auditing trauma care is to maintain quality assurance and to guide quality improvement. This study was conducted to identify the incidence, type, and setting of errors leading to morbidity and mortality in trauma patients. Determinations of the Medical Audit Committee of San Diego County were reviewed and classified by the authors for identification of preventable errors leading to morbidity or mortality. Errors were classified by type and categorized by phase of care. Errors were identified in the cases of 4% of all patients admitted for trauma care over a 4-year period. Of all trauma patient deaths, 5.9% were considered preventable or potentially preventable. The most common single error across all phases of care was failure to appropriately evaluate the abdomen. Although errors in the resuscitative and operative phases were more common, critical care errors had the greatest impact on preventable death. The detected error rate of 4% may represent the baseline error rate in a trauma system. While regionalized trauma care has dramatically reduced the incidence of preventable death after injury, efforts to further reduce preventable morbidity and mortality may be guided by an identification of common errors in a trauma system and their relationship to outcome.