A large body of research exists on the prevalence and impact of PTSD symptoms assessed in relation to a specific traumatic event, as well as on posttraumatic stress symptoms (PTSS) which refers to the presence of posttraumatic stress symptoms without measurement of a specific traumatic event. Migraine headache is one example of a moderately heritable chronic pain condition which has been found to co-occur with PTSD and PTSS. However, there is a dearth of scientific evidence to either support or refute any proposed models of co-occurrence. The specific aims of this study were to: 1) examine the extent to which shared genetic contributions convey a shared vulnerability to the association between PTSS and migraine headache symptoms (MHS); and 2) use longitudinal twin data to estimate the direction of the relationship between PTSS and MHS. Already available data, as of November 2013, was used from the University of Washington Twin Registry community-based sample of adult twin pairs. To address aim 1, 3,369 MZ and DZ pairs with complete data from the Registry at Time 1 were included in the analyses. To address aim 2, 1,134 MZ pairs with complete data from the Registry at both Time 1 and Time 2 were included in the analyses. A modest phenotypic association was found between PTSS and migraine headache for males and females. Bivariate analyses revealed that the proportion of the phenotypic association attributable to additive genetics that are common to both PTSS and MHS was an estimated 38% in males and 68% in females. The cross-lagged MZ twin difference model did not find PTSS at Time 1 to be directly related to MHS at Time 2 in both males and females or vice versa. Findings from both aims suggest that there is a modest overlap in genetic influences common to both PTSS and MHS in males and females, that this overlap is potentially more substantial in females, rather than one condition directly influencing the other. Understanding the underlying mechanisms that link PTSD or PTSS and chronic pain conditions such as migraine can provide insight into the development of tailored psychological and pharmacological interventions.