Studies on the incidence and epidemlologic features of brain injury and the immediate medical outcomes are few, and published results have serious methodological inconsistencies which prohibit comparisons. This study provides incidence rates of brain injury among the residents of San Diego, California. Cases had clinical confirmation and onset of injury occurred during 1981. The 3358 cases identified represent a rate of 180/100, 000 with males having a 2.2 times higher rate than females. Rates were highest for males aged 15–24 years and, for both genders, those over age 70. Forty-eight per cent of all cases were from transport-related causes, followed by falls (21%) and assaults (12%). Over 11% were dead-on-arrlval, and 16% were classified as having moderate or severe brain damage on admission to a hospital. Age- and sex-specific incidence rates varied according to external cause of injury. For example, for most subcategorles of motor vehicle crashes and for assaults, the incidence rate was highest among males aged 15–24, while for brain injuries from falls or firearms, highest incidence rates were observed in older age groups. Almost 7% of all cases discharged alive from an acute care hospital had significant neurologic sequelae. The impact of brain injury is discussed as a major unresolved public health problem.