BACKGROUND: Trauma is a major public health problem and organized systems of trauma care have been shown to substantially reduce trauma-related mortality. Currently California and many other states have incompletely developed systems of trauma care delivery. This study was undertaken to determine how frequently patients incurring serious trauma in California receive treatment at a trauma center. STUDY DESIGN: Hospital discharge records for 360,743 acute trauma patients for 1995 to 1997 were analyzed. Abbreviated Injury Scale scores were calculated from discharge diagnosis codes. Severity of trauma and the need for trauma center treatment was defined by eight Abbreviated Injury Scale criteria combined with patient age and type of injury. RESULTS: According to study criteria, 67,718 patients needed trauma center care and 56% were treated at a trauma center. Among patients less than 55 years of age, 62% were treated at a trauma center compared with 40% of those aged 55 years or more (p < 0.0001). For patients less than 55 years old with brain injuries, 66% were treated at a trauma center compared with 44% for patients aged 55 years or more (p < 0.0001). Of the 29,849 patients who met Abbreviated Injury Scale criteria but were not treated at trauma centers, 59% were in counties with designated trauma centers and 41% were in counties without trauma centers. CONCLUSIONS: Only 56% of seriously injured patients in California were treated at trauma centers, despite most of the injuries occurring in the catchment areas of designated trauma care systems. Substantial undertriage of serious trauma patients to trauma centers appears to be occurring, especially in older persons and in persons with brain injuries. Efforts to understand why undertriage is occurring so frequently are hampered by fragmentation of the systems of care, inadequate data management systems, and lack of trauma care performance reporting by non-trauma center hospitals.