In this study, the rate of remission, relapse, and treatment resistance in 107 patients with microscopic polyangiitis and necrotizing and crescentic glomerulonephritis associated with antineutrophil cytoplasmic autoantibodies were assessed. Patients with Wegener's granulomatosis were excluded. Prospective criteria were identified to assess remission, relapse, and resistant disease. Ninety-seven of the 107 patients received treatment with corticosteroids (N = 25) or with cyclophosphamide and corticosteroids (N = 72). Of these patients, 75 (77.3%) went into remission (complete remission, N = 61; remission on therapy, N = 14). Of the 75 responders, 32 patients (43%) remained in long-term remission, for a mean follow-up of 44 +/- 29 months; 15 patients (20%) progressed to ESRD without signs of relapse, for a mean of 21.4 +/- 22.8 months after the end of treatment; 6 patients died. Twenty-two of the 75 patients who initially responded to treatment (29%) suffered a relapse that occurred within 18 months of the end of therapy and usually affected the same organ systems as on initial presentation. There was a significant difference in the remission rate between the corticosteroid-treated patients and the cyclophosphamide-treated patients (56% versus 84.7%, P = 0.003), and the cyclophosphamide-treated patients had three times less risk of experiencing a relapse than did corticosteroid-treated patients (0.31, 95% Cl = (0.12, 0.84)). Seventy-seven percent (17 of 22 patients) of treatment resistance occurred in patients who presented with fulminant disease or advanced and severe renal disease. It was concluded that most patients with microscopic polyangiitis or necrotizing and crescentic glomerulonephritis achieve remission with therapy. Relapses occur in 29% of patients and generally respond to retreatment. Initial treatment with cyclophosphamide and corticosteroids rather than corticosteroids alone results in a lower frequency of relapse. Even patients who require dialysis at presentation may benefit from treatment, however, patients who are not treated until the disease process is life-threatening may die before induction therapy is complete, indicating the continued need for early diagnosis and therapy.