OBJECTIVE: To assess the efficacy of systemic corticosteroids alone as first-line treatment of polyarteritis nodosa (PAN) and microscopic polyangiitis (MPA) without poor-prognosis factors as defined by the Five-Factors Score (FFS), and to compare the efficacy and safety of azathioprine versus pulse cyclophosphamide as adjunctive immunosuppressive therapy for patients experiencing treatment failure or relapse. METHODS: This prospective, multicenter, therapeutic trial included 124 patients with newly diagnosed PAN or MPA (FFS of 0) treated with corticosteroids alone. At the time of treatment failure or disease relapse, patients were randomized to receive 6 months of therapy with oral azathioprine or 6 pulses of cyclophosphamide. Analyses was performed according to an intent-to-treat strategy. RESULTS: The mean +/- SD followup period was 62 +/- 33 months. Treatment with corticosteroids alone induced remission in 98 patients; 50 (40%) of these patients had sustained disease remission, 46 (37%) experienced a relapse, and 2 became corticosteroid dependent (daily prednisone dose ≥ 20 mg). In 26 patients (21%), treatment with corticosteroids alone failed, and 49 patients (40%) required additional immunosuppression. Among the 39 patients randomized, 13 of 19 achieved remission with cyclophosphamide pulses, and 14 of 20 achieved remission with azathioprine. Among all patients, the 1-year and 5-year survival rates were 99% and 92%, respectively. Six deaths occurred in the cyclophosphamide-treated group compared with 2 deaths in the azathioprine-treated group. Disease-free survival was significantly lower for patients with MPA than for those with PAN (P = 0.046). CONCLUSION: For patients with PAN or MPA with an FFS of 0, overall 5-year survival was good, but first-line corticosteroid treatment was able to achieve and maintain remission in only about half of the patients, and 40% of the patients required additional immunosuppressive therapy. Azathioprine or pulse cyclophosphamide was fairly effective for treating corticosteroid-resistant disease or major relapses.