OBJECTIVE: To compare immediate computed tomography during triage for admission with observation in hospital in patients with mild head injury. DESIGN: Multicentre, pragmatic, non-inferiority randomised trial. SETTING: 39 acute hospitals in Sweden. PARTICIPANTS: 2602 patients (aged > or = 6) with mild head injury. INTERVENTIONS: Immediate computed tomography or admission for observation. MAIN OUTCOME MEASURE: Dichotomised extended Glasgow outcome scale (1-7 v 8). The non-inferiority margin was 5 percentage points. RESULTS: At three months, 275 patients (21.4%) in the computed tomography group had not recovered completely compared with 300 (24.2%) admitted for observation. The difference was - 2.8 percentage points, non-significantly in favour of computed tomography (95% confidence interval - 6.1% to 0.6%). The worst outcomes (mortality and more severe loss of function) were similar between the groups. In the patients admitted for observation, there was a considerable delay in time to treatment in those who required surgery. None of the patients with normal findings on immediate computed tomography had complications later. Patients' satisfaction with the two strategies was similar. CONCLUSIONS: The use of computed tomography in the management of patients with mild head injury is feasible and leads to similar clinical outcomes compared with observation in hospital. TRIAL REGISTRATION: ISRCTN81464462.