 Posttraumatic stress disorder PTSD is a condition of critical importance to the Veterans Health Administration that affects up to 15 of Vietnam combat veterans.1 Studies in animals demonstrated that stress is associated with alterations in the morphology of the hippocampus, a brain structure involved in learning and memory, as well as memory deficits. Patients with PTSD have smaller hippocampal volume on magnetic resonance imaging MRI and deficits in memory on neuropsychological testing. Animal models also show a progressive diminution of hippocampal volume with normal aging that is associated with a loss of memory function. In humans, in very old age in some individuals the hippocampus becomes smaller, with associated declines in memory function. Clinically, PTSD patients appear to have a more rapid rate of memory decline with age, which has led to their being characterized as having an accelerated aging; however there is little empiric data related to the interaction between normal aging and PTSD and its effect on memory. In the prior period of VA Merit funding the PI has studied memory and the hippocampus in twin pairs discordant for PTSD as well as normal twin pairs. We have studied 42 MZ and DZ twin pairs discordant for Vietnam theater exposure and discordant for PTSD and 28 MZ and DZ twin pairs concordant for Vietnam theater exposure and discordant for PTSD. All twins underwent measures of hippocampal volume, neuropsychological testing, and cortisol. A subset of 10 pairs of MZ pairs discordant for PTSD and combat exposure underwent positron emission tomography PET imaging of neural correlates of declarative memory function. Preliminary analyses are consistent with smaller hippocampal volume and deficits in verbal declarative memory in PTSD affected twins compared to their non-PTSD brothers who were not exposed to Vietnam combat. 