This article summarises recent evidence that suggests that synaesthesia is one of the largest known risk factors for the development of the post-traumatic stress disorder (PTSD). This important and novel finding is explained in terms of the underlying cognitive differences that are found in people with synaesthesia. When asked to recall previous (non-traumatic), events, synaesthetes are more likely to report re-experiencing sensory and affective details from the time of the event and are more likely to report reliving the event from a first-person perspective. These memory qualities, perhaps coupled with memory inflexibility, may act as a clinical vulnerability to flashbacks following exposure to trauma.