The purposes of this study were to provide a national estimate of the incidence of traumatic brain injuries (TBIs) seen in emergency departments (EDs), but not requiring hospitalization and to determine the causes of these injuries. Using the Centers for Disease Control and Prevention case definition of TBI, ED data was analysed from the National Hospital Ambulatory Medical Care Survey (1995-1996). The average overall incidence rate of TBI-related ED visits for persons who were not hospitalized was 392/100,000 population per year, or 1,027,000 visits to hospital EDs in the US each year. This estimate is nearly twice (392 vs. 216) the previously estimated incidence rate, which was based on data from the 1991 National Health Interview Survey Injury Supplement. It was found that the highest incidence rate occurred among children aged 0-14 years, the rate for males was higher than for females, and the primary reported causes of these injuries were 'falls', motor vehicle-related causes, and 'struck by an object'. Although often considered 'mild' TBIs, these injuries can lead to significant cognitive and emotional impairment. Thus, continued surveillance of TBI-related ED visits is an important part of a comprehensive TBI prevention strategy.