This study investigated lifetime prevalence of traumatic events and posttraumatic stress disorder (PTSD) symptoms among 937 college students. Participants rated their lifetime experiences of traumatic events and, in response to their "most stressful" event, completed measures of objective stressor dimensions, PTSD, and peritraumatic reactions. Approximately 67% of respondents reported at least one traumatic event. An estimated 4% of the full sample (12% of traumatized individuals) met PTSD criteria within the past week. After controlling for vulnerability factors and objective characteristics, peritraumatic reactions remained strongly predictive of PTSD symptoms. Results are discussed with respect to immediate reactions to traumatic events as potential precursors of PTSD symptomatology.