The kidneys are frequently affected by systemic vasculitides. This is not surprising given the numerous vessels within the renal parenchyma. The kidneys are most often involved by small vessel vasculitides, such as microscopic polyangiitis (microscopic polyarteritis), Wegener's granulomatosis, Henoch-Schönlein purpura, and cryoglobulinemic vasculitis. These vasculitides cause renal dysfunction predominantly by inducing glomerular inflammation with resultant nephritis and renal failure. Microscopic polyangiitis (microscopic polyarteritis) and Wegener's granulomatosis are associated with and may be caused by antineutrophil cytoplasmic autoantibodies. Henoch-Schönlein purpura is caused by immunoglobulin (Ig) A-dominant immune complex localization in small vessels. Cryoglobulinemic vasculitis is sometimes induced by hepatitis C infection. Necrotizing medium-sized vessel vasculitides, such as classic polyarteritis nodosa and Kawasaki's disease, are less frequent causes of renal disease. They cause infarction secondary to thrombosis of inflamed major extrarenal and intrarenal arteries, and may lead to life-threatening hemorrhage from rupture of aneurysms. Large vessel vasculitides, such as giant cell (temporal) arteritis and Takayasu arteritis, only rarely injure the kidneys, usually by ischemia secondary to vasculitic involvement of the renal arteries or abdominal aorta. This ischemia can cause renovascular hypertension.