Focal and confluent areas of periventricular hyperintensity have been reported on magnetic resonance (MR) images in 30% of patients over 60 years of age. In order to better understand the pathologic basis of these lesions, the authors studied 14 formalin-fixed brains with MR imaging. Multiple focal areas of hyperintensity were identified in the periventricular white matter in three of the 14 brains studied (21%). Subsequent gross and microscopic pathologic examination of both hyperintense and normal-intensity areas was performed on 87 tissue sections. The larger lesions were characterized centrally by necrosis, axonal loss, and demyelination and therefore represent true infarcts. Reactive astrocytes oriented along the degenerated axons were identified at distances of up to several centimeters from the central infarct. This is called isomorphic gliosis and is associated with increased intensity on T2-weighted images that increases the apparent size of the central lesion.