Magnetic resonance (MR) imaging features of white matter lesions, often seen in the elderly, are correlated with histologic findings. Dilatation of perivascular spaces is seen, especially in the frontal and/or parietal subcortical white matter; the spaces are less than 3 mm in diameter and have sharp margins with no perifocal abnormality. Old lacunar infarcts are larger than 3 mm in diameter and are irregularly shaped and accompanied by perifocal myelin pallor and gliosis. Periventricular hyperintensity, including cap and rim, histologically shows myelin pallor, dilatation of perivascular spaces, discontinuity of the ependymal lining, and subependymal gliosis. Deep and subcortical white matter hyperintensity reflects myelin pallor and dilatation of perivascular spaces. Diffuse white matter lesion, seen in Binswanger's disease, shows myelin pallor and tissue rarefaction associated with loss of myelin and axons. U-fibers are usually well preserved. Severe arteriosclerosis and arteriolosclerosis are usually seen in the white matter. Knowledge of the pathologic features of incidental changes in white matter helps in understanding MR imaging findings.