The purpose of this study was to assess temporal lobe white matter changes accompanying hippocampal sclerosis on magnetic resonance (MR) imaging using single-voxel 1 H MR spectroscopy and to strengthen the hypothesis that these white matter changes are caused by myelin alterations. In 11 patients with histologically proven hippocampal sclerosis, preoperative coronal fluid-attenuated inversion recovery images were visually assessed by two experienced neuroradiologists for hippocampal signal increase and size decrease, atrophy of collateral white matter, and temporal lobe gray/white matter demarcation loss. Single-voxel 1 H MR spectroscopy of the white matter of each anterior temporal lobe was also performed, excluding the amygdala and hippocampus. The N-acetyl-aspartate (NAA)/choline and NAA/creatine ratios were calculated. In 12 healthy volunteers both temporal lobes were spectroscopically examined. In all patients the excised hippocampi were histologically assessed for the presence of sclerosis, and the excised neocortical temporal lobes were examined for gray and white matter abnormalities. MRI abnormalities were found on the right in six patients, on the left in four, and one scan was normal. Hippocampal signal increase was seen in nine patients, hippocampal size decrease in ten, atrophy of collateral white matter in nine, and gray/white matter demarcation loss in six. A significant decrease in the NAA/choline ratio was found in temporal lobe white matter ipsilateral to the pathologic hippocampus (symptomatic side), compared with the contralateral, asymptomatic side (P F 0.01), and also compared with controls (P F 0.001). The ipsilateral NAA/creatine ratio was also significantly decreased (P F 0.05) compared with the contralateral side and the control subjects (P F 0.001). Histological examination showed hippocampal sclerosis to a different degree in all patients. Neither gliosis nor cortical dysplasia was found in the ipsilateral, symptomatic temporal lobe. Significant decrease in the mean of NAA/choline ratios is found in temporal lobe white matter of patients with histologically confirmed hippocampal sclerosis. As this indi-cates neuronal loss or dysfunction, the number of axons may be reduced, with associated decrease in myelin density. J.