The role of vascular disorders in the pathogenesis of dementia has been controversial. Recent studies suggest that subcortical arteriosclerotic encephalopathy (Binswanger's disease), a disorder of white-matter demyelination associated with narrowing of penetrating medullary arteries and arterioles may affect more than 5% of the population over age 65 years. In part I of this paper, the authors present clinical examples and review the clinical literature, including clinical course and radiologic features. Differential diagnosis and treatment options are reviewed. In part II of this paper we will discuss theories of pathogenesis of subcortical arteriosclerotic encephalopathy and implications for the nosology of dementia.