Previous magnetic resonance (MR) volume imaging and proton MR spectroscopy studies have suggested a reduction in the hippocampal size and/or neuronal/axonal density in posttraumatic stress disorder (PTSD). The lack of agreement on the laterality of the hippocampal dysfunction prompted this study. A total of 20 veterans (18 men and two women) and one female non-veteran participated in this study conducted in accordance with approved human study protocols. Six of the male veterans and the female non-veteran were without PTSD. Vietnam veterans formed a large subset of the study subjects. Single-voxel proton MR spectra were obtained from the hippocampal region bilaterally on a clinical MR scanner operating at 1.5 T. Analysis of the proton MR spectra showed a decrease in hippocampal NAA/creatine ratio in PTSD subjects significantly higher on the left than the right for the entire study group, as well as for the Vietnam subset. It was concluded that the hippocampal dysfunction in PTSD is lateralized with the left side being more impaired than the right.