Understanding the mechanisms underlying the development of violence exposure sequelae is essential to providing effective treatments for traumatized youth. This longitudinal study examined the mediating role of posttraumatic stress in the relationship between violence exposure and psychopathology, and compared the mediated models by gender. Urban adolescents (n=1,358) were surveyed using the Social and Health Assessment. The proposed relationships were examined using Structural Equation Modeling. Posttraumatic stress fully mediated the relationships between victimization and depression and anxiety in girls, and partially so in boys. In addition, posttraumatic stress partially mediated the relationships between violence exposure and commission of violence in boys. Current findings support the longitudinal effects of violence exposure on adolescent mental health. Posttraumatic stress represents a unique mechanism for the development of psychopathology in girls and is also related to negative outcomes in boys. These findings have direct implications for prevention and rehabilitation efforts among violence exposed youth.