The 1991 National Health Interview Survey was analysed to describe the incidence of mild and moderate brain injury in the United States. Data were collected from 46 761 households and weighted to reflect all non-institutionalized civilians. The report of one or more occurrences of head injury resulting in loss of consciousness in the previous 12 months was the main outcome measure. Each year an estimated 1.5 million non-institutionalized US civilians sustain a non-fatal brain injury that does not result in institutionalization, a rate of 618 per 100,000 person-years. Motor vehicles were involved in 28% of the brain injuries, sports and physical activity were responsible for 20%, and assaults were responsible for 9%. Medical care was sought by 75% of those with brain injury; 14% were treated in clinics or offices, 35% were treated in emergency departments, and 25% were hospitalized. The risk of medically attended brain injury was highest among three subgroups: teens and young adults, males, and persons with low income who lived alone. The incidence of mild and moderate brain injury in the United States is substantial. The National Health Interview Survey is an important national source of current outpatient brain-injury data.