Objective: Adolescents constitute a high-risk population for traumatic physical injury, yet few longitudinal investigations have assessed the development of posttraumatic stress disorder (PTSD) symptoms over time in representative samples. Method: Between July 2002 and August 2003,108 randomly selected injured adolescent patients ages 12 to 18 and their parents were interviewed at baseline and again 2, 5, and 12 months postinjury. Initially, participants were screened for PTSD symptoms with the PTSD Reaction Index (PTSD-RI) and depressive symptoms with the Center for Epidemiologic Studies Depression Scale, as well as preinjury trauma. Random-coefficient regression was used to assess the association between baseline clinical, injury, and demographic characteristics and the development and maintenance of PTSD symptoms longitudinally. Results: Between 19% and 32% of adolescents screened positive for PTSD (i.e., had PTSD-RI scores of [greater than or equal to]38) during the course of the 12 months after the injury. Higher initial adolescent PTSD and depressive symptoms, higher emergency department heart rate, greater objective event severity, and greater parental preinjury trauma were significant independent predictors of higher adolescent PTSD symptoms. Conclusions: For a substantive minority of hospitalized adolescents, high PTSD symptom levels persist during the 12 months after injury. Clinical characteristics readily identifiable after the acute injury predict the development of PTSD symptoms over time. Real-world clinical trials that test screening and intervention procedures for representative samples of at-risk youths are warranted. (Contains 1 figure and 3 tables.)