Recently individualized from polyarteritis nodosa (PAN), microscopic polyangiitis (MPA) is defined as a systemic necrotizing vasculitis that clinically and histologically affects small-sized vessels (ie, capillaries, venules or arterioles) without granulomata and is associated with focal segmental necrotizing glomerulonephritis. Males are more frequently affected than females and the average age of onset is about 50 years old. Most patients experience some systemic symptoms before diagnosis of vasculitis. Clinically, renal involvement is the major feature of MPA and is characterized by rapidly progressive glomerulonephritis (RPGN). Most of the patient have renal impairment at admission and renal function deteriorates rapidly without treatment. Lung involvement is also common. Lung hemorrhage is observed in 12 to 29% of the patients with MPA and is an important contributory factor to morbidity and mortality. Some patients with small-vessel lung vasculitis may present clinical, radiologic and functional findings consistent with an interstitial process mimicking idiopathic pulmonary fibrosis. Others clinical features are similar to those observed in PAN. Musculoskeletal involvement (myalgias, arthralgias and arthritis) are present in 65 to 72% of the patients. Cutaneous lesions (purpura, splinter hemorrhages) are found in 44 to 58% of the patients. Gastrointestinal symptoms are characterized by abdominal pain (32 to 58%) and digestive tract bleeding (29%). Peripheral neuropathy is found in only 14 to 36% of the cases, thus occurring less frequently than in PAN. Ocular manifestations and ear, nose and throat lesions are commonly seen, more frequently than in PAN. Non-specific laboratory tests reflect the systemic inflammatory nature. Almost all patients are negative for hepatitis B virus (HBV) surface antigen. Renal insufficiency with creatininemia > 120 microns/l is present in the majority of patients. Antineutrophil cytoplasm antibodies (ANCA) are found in 75% of MPA patients and the majority of these ANCA detected are perinuclear-staining anti-myeloperoxidase ANCA, although anti-proteinase 3 has also be detected. Microaneurysms, commonly present in PAN, are rarely seen on at visceral angiograms. MPA is part of a spectrum of systemic vasculitides. Differentiation between PAN and MPA should be based on clinical manifestations (especially lung and kidney involvement), biologic signs (ANCA, HBV or HCV infection) and angiographic data. The therapeutic strategies for treatment of PAN and MPA do not differ extensively. Prognosis of systemic vasculitides have been transformed by corticosteroids that are the basis of the treatment. Immunosuppressive drugs, especially cyclophosphamide, also contribute to a better prognosis. Considering the high frequency of renal involvement in MPA, most of the patients should considered as having factors or poor prognosis and the high number of relapses that can occur in patients with MPA could justify prolonged steroid administration or immunosuppressive treatment.