Background-The pathogenesis of and relationship between small deep (lacunar) infarcts, cerebral white matter disease (leukoaraiosis or white matter hyperintensities), and progressive cognitive impairment or dementia are much debated. Summary of Comment-We hypothesize that cerebral small-vessel endothelial (ie, blood-brain barrier) dysfunction, with leakage of plasma components into the vessel wall and surrounding brain tissue leading to neuronal damage, may contribute to the development of 3 overlapping and disabling cerebrovascular conditions: lacunar stroke, leukoaraiosis, and dementia. This hypothesis could explain the link between ischemic cerebral small-vessel disease and several apparently clinically distinct dementia syndromes. This hypothesis is supported by pathological, epidemiological, and experimental studies in lacunar stroke and leukoaraiosis and observations on the blood-brain barrier with MRI. We suspect that the potential significance of blood-brain barrier failure as a pathogenetic step linking vascular disease with common, disabling brain diseases of insidious onset has been overlooked. For example, lipohyalinosis, which has a pathological appearance of uncertain origin and is possibly responsible for some discrete lacunar infarcts, may be one end of a clinical spectrum of illness manifested by blood-brain barrier failure. Conclusions-Proof that blood-brain barrier failure is key to these conditions could provide a target for new treatments to reduce the effects of vascular disease on the brain and prevent cognitive decline and dementia.