Abstract: This study estimates the cumulative occurrence of traumatic events and Posttraumatic Stress Disorder (PTSD) using Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV) criteria, in a high-risk sample of youth in urban city of Lagos, Nigeria. Participants were interviewed about history of trauma and PTSD in 2006-2007 when their mean age was 16 years (n = 1,702). The study revealed that the lifetime occurrence of assaultive violence was 62.5% in males and 33.6% in females. Females had a higher risk of PTSD than males following assaultive violence (odds ratio = 4.0, 95%) confidence interval (2.0-8.3) but not following other traumas. A comparison of the results from this heterogeneous sample of youth from urban city of Lagos with the results from study of suburban sample in United States in which the same criteria and measures of trauma and PTSD were used suggested the possibility that males’ risk for assualtive violence and females’ risk for PTSD following exposure to assaultive violence might vary by characteristics of the environment especially social environment.