We studied 17 patients with severe systemic necrotizing vasculitis over an 11-year-period. Sixteen patients were treated daily with cyclophosphamide (2 mg per kilogram per day), and one was treated with azathioprine (2 mg per kilogram per day). Before entering the study, all patients had active and progressive disease, even though 16 patients had been receiving corticosteroids that had caused severe and often incapacitating toxic side effects. Three patients died during the study. Complete and often dramatic remissions occurred in the surviving 14 patients, who were then placed on alternate-day corticosteroid treatment with continuation of cyclophosphamide. Corticosteroids were later discontinued in seven patients. The mean duration of remission was 22 months (range, two to 61). No patient showed recurrence of disease during treatment with cytotoxic agents.