The determination that a particular head injury is "mild" or "low-risk" is usually made on clinical grounds. Observation at home or in the hospital has been the usual treatment for such patients. A recent report of excessive mortality among these patients with low-risk head injuries in some hospital settings suggests the need for improvement in diagnostic criteria. Our study included 658 patients admitted to Cooper Hospital for a mild head injury, who had a Glasgow Coma Scale score of 13 to 15 on admission, and who experienced a brief loss of consciousness or amnesia after the injury. Their records and cranial computed tomographic (CT) scans on admission were reviewed to determine their subsequent course. In 18% of patients abnormalities were seen on the initial CT scan and 5% required surgery. Among the 62 patients with a Glasgow Coma Scale score of 13, 40% had abnormalities on the CT scan and 10% required surgery. None of the 542 patients with normal CT scans on admission showed subsequent deterioration and none needed surgery. These figures suggest that history and physical examination alone are not adequate to assess head injury or severity of risk and that the addition of a CT scan greatly improves patient assessment. Abnormalities on CT scans are so common in patients with a Glasgow Coma Scale score of 13 that head injuries in these patients should be classified as "moderate" rather than "mild" in severity and risk. Patients with normal CT scans should be considered for observation at home, allowing hospital personnel to devote full attention to the more seriously injured patients.