The generally accepted definition of mild head injury includes Glasgow Coma Scale (GCS) scores of 13 to 15. However, many studies have shown that there is a heterogeneous pathophysiology among patients with GCS scores in this range. The current definition of mild head injury is misleading because patients classified in this category can have severe sequelae. Therefore, a prospective study of 1360 head-injured patients with GCS scores ranging from 13 to 15 who were admitted to the neurosurgery service during 1994 and 1995 was undertaken to modify the current definition of mild head injury. Data regarding patients' age, sex, GCS score, radiographic findings, neurosurgical intervention, and 6-month outcome were collected and analyzed. The results of this study showed that patients with lower GCS scores tended to have suffered more serious injury. There was a statistically significant trend across GCS scores for percentage of patients with positive acute radiographic findings, percentage receiving neurosurgical interventions, and percentage with poor outcome. The presence of postinjury vomiting did not correlate with findings of acute radiographic abnormalities. Based on the results of this study, the authors divided all head-injured patients with GCS scores ranging from 13 to 15 into mild head injury and high-risk mild head injury groups. Mild head injury is defined as a GCS score of 15 without acute radiographic abnormalities, whereas high-risk mild head injury is defined as GCS scores of 13 or 14, or a GCS score of 15 with acute radiographic abnormalities. This more precise definition of mild head injury is simple to use and may help avoid the confusion caused by the current classification.