Traumatic brain injury (TBI) is a leading cause of death and disability among United States adolescents. The authors sought to determine causes and trends for TBI-related hospitalizations in the United States adolescent population (10-19 years). The authors identified common causes and trends of adolescent TBI, overall and within 2-year age categories, using hospitalization data from 2005 to 2009 in the Nationwide Inpatient Sample. The leading cause of adolescent TBI overall was motor vehicle occupant accidents (35%), which are also the leading cause in the 14-15, 16-17, and 18-19 year age groups. Falls were the cause of most TBI in the 10-11 year (23%) and 12-13 year (20%) age groups. For both unintentional and intentional mechanisms of injury, there was evidence of increasing hospitalizations with increasing age. From 2005 to 2009, the overall annual incidence of adolescent TBI hospitalizations decreased 21% from an estimated 75.5-59.3 per 100,000 (p<0.001). These rates declined for mild, moderate, and severe TBI, and decreased for 2-year age groups, except for the 18-19 year-old group. For TBI attributable to motor vehicle occupants, rates declined 27% from 27.6 to 20.2 per 100,000 (p<0.001). Motor vehicle occupant injuries account for 42% of in-hospital mortality from adolescent TBI; however, firearms are the most lethal mechanism with 46% proportional mortality among victims of firearm-related TBI. Rates of adolescent TBI-related hospitalizations have decreased overall. Motor vehicle accidents and firearms were identified as leading causes of injury and mortality for adolescent TBI, and represent potential targets for intervention.