The value of antineutrophilic cytoplasmic antibodies (ANCA) was assessed in the diagnosis and chronic treatment of 7 patients with microscopic polyarteritis or Wegener's granulomatosis. All patients had oligoimmune glomerulonephritis with segmental and focal necrosis and presented with anaemia. Five of them had alveolar haemorrhage with haemoptysis and infiltrates at radiography. ANCA were assayed by indirect immuno-fluorescence on ethanol-fixed neutrophils and were strongly positive, with a cycloplasmic aspect in 5 cases and a perinuclear aspect in 2 cases. Initial remission with fall in ANCA titres was obtained with corticosteroids, cyclophosphamide and sometimes plasmapheresis (5 patients), but frequent relapses with re-elevation of ANCA titre occurred when treatment was reduced. It is concluded that ANCA are very helpful in the diagnosis of systemic vasculitis, notably in cases with first-time alveolar haemorrhage. They also facilitate monitoring and therapeutic decisions, since relapses are frequent.