Comparison of postmortem magnetic resonance imaging and neuropathologic findings in the cerebral white matter. Two types of high-signal intensity abnormalities are frequently found bilaterally in the cerebral white matter of brains of elderly patients on T2-weighted magnetic resonance imaging (MRI) scans. One is located in the immediate periventricular region; the other, in the deep subcortical white matter (centrum semiovale). The diagnostic implications of this second type continue to be uncertain. To determine the neuropathologic correlates of these lesions, the brains from seven elderly patients were fixed in buffered formaldehyde solution, subjected to MRI scanning, and examined neuropathologically. Variable degrees of bilateral periventricular (subependymal) sharply defined areas of high-signal intensity were found in all the brains, and the larger of these showed corresponding areas of myelin pallor with gliosis and dilated perivascular spaces. Discrete bilateral patches of high-signal intensity were found in the centrum semiovale in five patients. Myelin and axon stains showed varying degrees of diffuse white matter pallor in many areas examined, both with and without these areas of high-signal intensity on MRI scans. Neither the myelin nor the axon stains showed discrete white matter abnormalities that corresponded to the MRI findings. We believe that these changes, so commonly found on MRI scans in the elderly, reflect actual changes in the white matter but that their nature and clinical significance need to be elucidated. 