Background: Appropriate triage of the trauma patient is essential to ensure prompt access to definitive care. Many trauma centres use a “tiered” trauma call protocol with the intention of providing a match between the facility’s resources and the needs of the patient. This study describes the incidence and impact of undertriage on the trauma patient in the context of an Australian level 1 trauma centre with a tiered trauma call system. Methods: This was a retrospective analysis of prospective data collected through the Trauma Registry. Undertriage was defined as sustaining an injury severity score greater than 15 and receiving a non-optimal response (i.e., trauma standby call or no call). The level of association between outcome measures (such as LOS in ED, time to OT) and the level of trauma call the patient received was assessed using a general linear model, controlling for injury severity and haemodynamic stability. Results: Between February 2004 and November 2008, 5233 patients meeting trauma criteria presented to the study hospital. There was an undertriage rate of 42% and overtriage rate of 21%. Patients were more likely to be undertriaged if they were older, self-presented, their cause of injury was assault or their head or chest were their most severely injured body region. Undertriaged patients had a significantly longer LOS in the ED (2 h) than appropriately triaged patients. Conclusion: The implementation of a tiered trauma call system resulted in significant undertriage, especially if the patient was older, had been assaulted or had a head/chest injury. Undertriaged patients experienced delay to definitive care. This study has highlighted the importance of compliance with trauma team activation criteria, trauma monitoring and evaluation. © 2010 Published by Elsevier Ltd. All rights reserved.