Objective: This study analyzed predictors of post-traumatic stress disorder (PTSD) in civilian trauma victims to assess how peritraumatic dissociation (PD) relates to PTSD symptom development. We examined PD and PTSD symptoms from a prior trauma simultaneously to better understand the extent towhich past and current reactions to a trauma can predict the development of PTSD for a current trauma.Methods: Participants (N¼48) were recruited from the emergency department (ED) of a large, southeastern hospital and assessed immediately after a trauma and again at 4 weeks and 12 weekspost-trauma. We used both self-report and interviewer-based questionnaires to assess PD and PTSD symptoms for prior and current trauma.Results: A hierarchical linear regression revealed that at 4-week follow up, when controlling for several demographic variables and trauma type, a model including both PD and PTSD symptoms from a prior trauma significantly predicted PTSD outcome (F(47)¼3.70, p¼0.00), with PD and prior PTSD symptoms significantly contributing 17% and 9% of variance respectively. At 12 weeks, PTSD symptoms from prior trauma (b¼0.094, p¼0.538) and PD (b¼0.017, p¼0.909) did not account for a significant proportion of the variance in PTSD for the enrolling trauma.Conclusion: Prior and current reactions to trauma are both important factors in predicting the development of PTSD symptoms to a current trauma. The more immediate measurement of PD during presentation to the ED may explain the strength of its relationship to PTSD symptom development.Furthermore, our findings support the use of PTSD symptoms of a past trauma, as opposed to trauma frequency, as a predictor of PTSD from a subsequent trauma. Methodological limitations and future directions are discussed. [West J Emerg Med. 2012;13(3):220–224.]