Community violence and its physical health consequences are well known among youth living in urban settings. However, less is known about the cumulative effect of contextual and demographic risk factors on posttraumatic stress symptoms (PTSS) among vulnerable youth. Longitudinal data (baseline, 9-month, 21-month) were analyzed to investigate trajectories of PTSS, internalizing, and externalizing symptoms among 188 youth (M[subscript age] 12.87, 60.6% male) treated for an assault injury in an emergency department. Youth exhibited decreased mental health problems over time. Higher levels of internalizing symptoms related to decreased PTSS over time, while higher levels of PTSS predicted increased externalizing symptoms over time, thus underscoring the importance of understanding comorbidity. Gender and stressful life events were significantly associated with initial levels of symptoms and trajectories. These findings suggest the importance of understanding PTSS in the context of environments and personal factors to support appropriate treatment.