Objective : To determine the short-term cognitive and symptomatic outcome following mild traumatic brain injury. Methods : Setting: Emergency Departments of two UK tertiary referral hospitals. Participants: Adult patients presenting to the Emergency Departments of the Royal London Hospital and Salford Royal Hospital with suspected traumatic brain injury within 24 hours and Glasgow Coma Score > 8. A non-TBI comparison group included adult patients with no head or neck injury. Design: Prospective multi-centre cohort study. Main measures: The Standardized Assessment of Concussion (SAC), the Concussion Symptom Inventory (CSI) and total number of symptoms, measured at baseline and 72 hours. Results : This study enrolled 189 patients with and 51 patients without TBI. Patients with TBI had marked cognitive impairment which persisted at 72 hours (SAC score at baseline = 25 [23–27] vs 72 hours = 25 [22–27]; p = 0.1). Patients with TBI had persistent high symptom severity, although this had decreased at 72 hours (CSI score at baseline = 9 [4–22] vs 72 hours = 5 [1–19], p = 0.002). A similar pattern was observed with the total number of symptoms (baseline = 4 [2–8] vs 72 hours = 0 [0–4]; p