Objective: Posttraumatic stress disorder (PTSD) is one of the only disorders in the Diagnostic and Statistical Manual of Mental Disorders that requires an environmental exposure. The relationship between liability factors for trauma exposure and those for PTSD symptoms following exposure are unclear. Methods: Exposure to a trauma and resulting PTSD symptoms were assessed in a sample of 2,794 members of the Norwegian Institute of Public Health Twin Panel. Results: In the full sample, 737 twins experienced a trauma. A modified causal, contingent, common pathway model was used to examine trauma exposure and liability for PTSD. Genetic and common environmental factors could not be distinguished, so a model that included only familial and individual specific components was fit. The best-fitting model suggested that familial factors played an important role in liability for trauma exposure and for resulting PTSD symptoms, and that there was a modest transmission between trauma exposure and subsequent PTSD symptoms. Conclusions: One third of the variance in liability of PTSD symptoms is due to familial factors, and of this, approximately one fifth overlaps with the familial liability for trauma exposure while the other four fifths of the variance is specific to the risk of PTSD symptoms following exposure. The hypothesis that PTSD is etiologically similar to exposures to a traumatic event is not supported, suggesting that the factors that confer risk for trauma do not overlap completely with those that confer risk for PTSD.