The emergence of sensitive techniques for brain imaging has drawn attention to the occurrence of diffuse or multifocal changes affecting the cerebral white matter. The white matter changes are usually termed periventricular leukoencephalopathy, or leukoaraiosis. Microscopic studies of affected areas in the deep white matter have shown mostly demyelination, reactive gliosis, and arteriolosclerosis, proportional to the degree of radiologic changes. Yet, many other disease conditions need to be ruled out. Risk factors for ischemic leukoaraiosis include arterial hypertension, a history of stroke, and age. In the hereditary disorder CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy), severe white matter changes occur in the absence of hypertension. In "ordinary" cases of leukoaraiosis, genetic factors might similarly determine the effect of risk factors on the aging brain and might explain, for example, why not all patients with severe hypertension develop leukoaraiosis. Not surprisingly, diffuse demyelination affects cognitive function. Although reduced speed of mental processes is the most characteristic sign, attention, concentration, and verbal and visual memory are also affected. Most importantly, less severe forms of cognitive impairment represent a silent and perhaps largely preventable epidemic among aged or even middle-aged subjects. They live independently, but mentally they perform on a level well below their previous capacities. Although being "a bit odd" does not lead to hospital admissions, it seriously affects quality of life of a large part of the community. Moderate grades of leukoaraiosis constitute a major public health problem and deserve the attention of the scientific community.