This chapter explores the complex relationship of stroke with polyarteritis nodosa (PAN) and microscopic polyangiitis (MPA). MPA is a systemic necrotizing vasculitis that clinically and histologically involves capillaries, venules, or arterioles without granulomata, and is associated with necrotizing crescentic glomerulonephritis and hemorrhagic pulmonary capillaritis, which are the main causes of mortality and morbidity. Hemorrhagic strokes occur more frequently than ischemic infarction in MPA. Immunohistochemical studies from muscle and nerve biopsies showed that macrophages and T cells, mostly CD8+, are involved in the pathogenesis of PAN. Neurological symptoms and signs are a major and common feature of PAN, occurring in nearly three-quarters of patients. A close relationship between the use of corticosteroids and stroke exists in PAN. From a therapeutic point of view, antiplatelet drugs, which inhibit platelet thromboxane production, might reduce the risk of corticosteroid-induced, antiplatelet drugs in PAN. The use of aspirin and corticosteroids prospectively prevents stroke recurrence.