Abstract Background Studies in animals showed that the hippocampus, a brain area involved in learning and memory, is sensitive to stress. Although several MRI studies showed smaller hippocampal volume in adults with chronic PTSD, others did not show significant differences from controls. These studies are typified by small sample sizes which may limit the ability to show significant differences. We therefore performed a meta-analytic study of all of these studies to clarify the role of hippocampal structural changes in subjects with PTSD. Methods Nine studies with a total of 133 adult subjects with chronic PTSD, 148 healthy controls, and 53 traumatized controls were included in the meta-analysis. Results There was significantly smaller volume in both right and left hippocampi in adult subjects with chronic PTSD in comparison with both healthy controls and traumatized controls. Conclusion These findings are consistent with smaller hippocampal volume in adult subjects with chronic PTSD.