 We now propose to measure hippocampal volume, memory function, and neural correlates of declarative memory, in MZ and DZ twins previously studied, who are discordant for combat and PTSD. We will study 42 twin pairs who were studied during 1999-2003, of whom 34 were discordant for PTSD at the Atlanta based assessment and had all study data including MRI, neuropsychological testing, and cortisol with PET in a subsample of 10 pairs, with repeat measurements performed 8 years after the initial assessment, a time of life when they become vulnerable to memory decline associated with aging subjects not discordant for PTSD at the Atlanta based assessment will also be studied as a comparison. These assessments will allow a comprehensive assessment of the relative impact of shared environment e.g. family life, unique environment Vietnam theater service, and genetics on the outcomes of interest. We hypothesize that there will be an interaction between PTSD diagnosis and aging, i.e. PTSD patients relative to non-PTSD will show smaller hippocampal volume as measured with MRI, deficits in verbal declarative memory as measured with neuropsychological testing, lower cortisol, and a failure of hippocampal function during memory tasks as measured with PET, and that the rate of decline will be greater in PTSD v non PTSD. We further hypothesize that analysis of all twin pair combinations will show a combined impact of genetics and PTSD diagnosis on the hippocampal outcomes of interest. 