Trauma centers save lives, but they are scarce and concentrated in urban settings. The population of severely injured children in California who do not receive trauma center care (undertriage) is not well understood.Retrospective observational study of all children (0-17 years) hospitalized for severe trauma in California (2005-2015). We used the California Office of Statewide Health Planning and Development linked Emergency Department and Inpatient Discharge data sets. Logistic regression models were created to analyze characteristics associated with undertriage. The model was clustered on differential distance between distance from residence to primary triage hospital and distance from residence to nearest trauma center. We controlled for body part injured, injury type, intent and year. The a priori hypothesis was that uninsured and publicly insured children and hospitals and regions with limited resources would be associated with undertriage.Twelve percent (1866/15 656) of children with severe injury experienced undertriage. Children aged >14 years compared with 0-13 years had more than 2.5 times the odds of undertriage (OR 2.58; 95%?CI 2.1 to 3.16). Children with private Health Maintenance Organization (HMO) insurance compared with public insurance had 13 times the odds of undertriage (OR 12.62; 95%?CI 8.95 to 17.79). Hospitals with >400 compared with <200 beds had more than three times the odds of undertriage (OR 3.64; 95%?CI 2.6 to 5.11). Urban versus suburban residence had 1.3 times increased odds of undertriage (OR 1.31; 95%?CI 1.02 to 1.67) Undertriage volume was largest in urban areas.Undertriage is associated with private HMO insurance, primary triage to large hospitals and urban residence. Understanding the characteristics associated with undertriage can help improve trauma systems.Level III (non-experimental retrospective observational study).