The lack of a common, widely acceptable criterion for the definition of trivial, minor, or mild head injury has led to confusion and difficulty in comparing findings in published series. This review proposes that acute head-injured patients previously described as minor, mild, or trivial are defined as "mild head injury," and that further groups are recognized and classified as "low-risk mild head injury," "medium risk mild head injury," or "high-risk mild head injury." Low-risk mild injury patients are those with a Glasgow Coma Score (GCS) of 15 and without a history of loss of consciousness, amnesia, vomiting, or diffuse headache. The risk of intracranial hematoma requiring surgical evacuation is definitively less than 0.1:100. These patients can be sent home with written recommendations. Medium risk mild injury patients have a GCS of 15 and one or more of the following symptoms: loss of consciousness, amnesia, vomiting, or diffuse headache. The risk of intracranial hematoma requiring surgical evacuation is in the range of 1-3:100. Where there is one computed tomography (CT) scanner available in an area for 100,000 people or less, a CT scan should be obtained for such patients. If CT scanning is not so readily available, adults should have a skull x-ray and, if this shows a fracture, should be moved to the "high-risk" category and undergo CT scanning. High-risk mild head injury patients are those with an admission GCS of 14 or 15, with a skull fracture and/or neurological deficits. The risk of intracranial hematoma requiring surgical evacuation is in the range 6-10:100. If a CT scan is available for 500,000 people or less, this examination must be obtained. Patients with one of the following risk factors--coagulopathy, drug or alcohol consumption, previous neurosurgical procedures, pretrauma epilepsy, or age over 60 years--are included in the high-risk group independent of the clinical presentation.