Background: Assessment of patients with mild traumatic brain injury (mTBI) is predominantly done using the Glasgow Coma Scale (GCS). While the GCS is universally accepted for assessment of severity of traumatic brain injury, it may not be appropriate to rely on the GCS alone when assessing patients with mTBI in prehospital settings and emergency departments. Objective: To determine whether administering the Revised Westmead Post-traumatic Amnesia (PTA) Scale (R-WPTAS) in addition to the GCS would increase diagnostic accuracy in the early identification of cognitive impairment in patients with mTBI. Methods: Data were collected from 82 consecutive participants with mTBI who presented to the emergency department of a level 1 trauma centre in Australia. A matched sample of 88 control participants who attended the emergency department for reasons other than head trauma was also assessed. All patients were assessed using the GCS, R-WPTAS and a battery of neuropsychological tests. Results: Patients with mTBI scored poorly compared with control patients on all measures. The R-WPTAS showed greater concurrent validity with the neuropsychological measures than the GCS and significantly increased prediction of group membership of patients with mTBI with cognitive impairment. Conclusions: The R-WPTAS significantly improves diagnostic accuracy in identifying patients with mTBI who may be in PTA. Administration takes less than 1 min, and since early identification of a patient's cognitive status facilitates management decisions, it is recommended for routine use whenever the GCS is used.