Lacunar infarcts and "hypertensive" primary intracerebral hemorrhages, collectively often referred to as hypertensive small vessel strokes, constitute about one third of all strokes. However, despite their public health importance, their etiopathogenesis remains ill-understood. Like all strokes, they are a heterogeneous entity, but the autopsy pathology evidence suggests that the majority are caused by a limited number of cerebral small vessel lesions. Small vessel atherosclerosis is causally implicated in a proportion of lacunar infarcts, although modern concepts of atherosclerotic plaque biology and natural history have yet to be applied to small cerebral vessels. A lesion characterized in its acute form by fibrinoid necrosis appears to be important in causing both lacunar infarcts and primary intracerebral bleeds. Advances in molecular genetics may prove instrumental in understanding the cause of this lesion and therefore in designing its targeted prevention.