Objective: Atrophic changes in the hippocampus that maybe seen during the late stage of traumatic brain injury (TBI) has been found to be highly related to the cognitive function. Previous studies detected by MR morphometric analysis have described atrophic changes in the hippocampus in patients, particularly with no hippocampal lesion in MRI. Use of different imaging parameters and quantification techniques may result in different results and we therefore used MRS to investigate the neuronal loss of the hippocampus. The goal of this study was to show the neuronal loss of hippocampus at the late stage of trauma, with hippocampal MRS in patients with a normal posttraumatic hippocampal MRI. Methods: Proton magnetic resonance spectroscopy (MRS) was used to evaluate the hippocampus of 13 severe TBI patients. Five of the patients had already regained consciousness while eight were still in the persistent vegetative state (PVS). There were no hippocampal lesions seen in the MRI of the subjects. The patients were divided into two groups according to time interval after the TBI. Eight patients had been evaluated earlier than 12 months (Group A) and five had been evaluated after the first year (Group B). NAA/Cr rates were measured bilaterally by performing long Te single voxel proton spectroscopy of the hippocampus. The results were compared with those of five healthy volunteers (Group C). Results: Comparing Group B and C, there was a significant decrease in the NAA/Cr ratio (p<0.01) for both the right and left hippocampus. However, when Groups A and C were compared, the results were only statistically significant for the left side (p<0.01). There was no relation between the NN/Cr ratio and the Glasgow Outcome Scale (GOS). Conclusion: MRS can be an indicator of neuronal loss in the hippocampus after severe TBI. The significance of the results in the late posttraumatic period is an indicator of hippocampal atrophy.