Background and Objectives: There is growing research indicating that event centrality strongly predicts posttraumatic stress disorder (PTSD). However, there is not much research on what makes a traumatic event central to one's life story. One reason a traumatic memory becomes a reference point for one's life story is that it brings about significant changes in one's life. This study investigated the relationships between transitional impact of an event, event centrality, and PTSD symptoms. Specifically, it tested whether higher negative changes were associated with higher levels of event centrality and PTSD symptoms. It also investigated whether event centrality mediated the relationship between transitional impact and PTSD severity. Methods: 101 individuals diagnosed with PTSD completed the Transitional Impact Scale (TIS), Centrality of Event Scale, and Impact of Event Scale-Revised regarding their traumatic experiences. Furthermore, they responded to the Posttraumatic Stress Disorder Symptom Scale-Self Report (PSS-SR), Traumatic Events Checklist (TELC), and Beck Anxiety Inventory (BAI). Results and Conclusions: Results indicate that (a) the TIS had valid and reliable psychometric properties in a PTSD sample, (b) most of the traumatic events received high negative psychological and material change scores, and (c) event centrality mediated the relationship between transitional impact and PTSD severity.