Microscopic polyangiitis is a non-granulomatous necrotizing vasculitis involving small vessels. Clinical manifestations are highly polymorphic, but rapidly progressive glomerulonephritis is one of the most frequent and most severe manifestations of the disease. Biopsy of an affected organ and detection of circulating anti-neutrophil cytoplasmic antibodies (ANCA) are key elements for the positive diagnosis of microscopic polyangiitis. Biopsies can disclose necrotizing vasculitis affecting small vessels, without granulomas and without immune deposits. ANCA are very specific for microscopic polyangiitis, Wegener's granulomatosis and Churg-Strauss syndrome when they are positive by indirect immunofluorescence and are directed against myeloperoxidase or proteinase 3. Such ANCA are found in about 70% of patients with microscopic polyangiitis. Treatment of severe forms of microscopic polyangiitis is based on the administration of pulse methylprednisolone, oral corticosteroids and cyclophosphamide. In the mildest forms of the disease, one can probably try either to competely avoid using immunosuppressive drugs, or to replace cyclophosphamide with azathioprine. Treatment induces a complete remission of the disease in more than 90% of cases, but about 30% of the patients will experience a relapse, and progressive worsening of renal function can occur in patients with severe chronic renal failure.