The fear of failing to identify brain injury has led to the liberal and potentially excessive use of computed tomographic (CT) scanning of patients with blunt head trauma who have even a remote possibility of intracranial injury. This practice exposes large numbers of patients to the expense and radiation exposure associated with CT imaging while detecting injuries in a small minority. Previous studies suggest that it might be possible to develop a decision instrument to identify patients with blunt head injury who have essentially no risk of significant intracranial injury and for whom CT scanning is therefore unnecessary. Development of such a decision instrument has been identified as a priority among practicing emergency physicians. The National Emergency X-Radiography Utilization Study II (NEXUS II) is a large, multicenter, prospective study designed to derive a decision rule for CT imaging of patients with blunt head injury. This study, conducted in 21 different emergency departments across the United States and Canada, will enroll more than 10 times as many patients with head trauma as any currently published study. NEXUS II should be able to definitively answer questions about the validity and reliability of clinical criteria as a preliminary screen for blunt head trauma.