We (JMAAV [Japanese patients with MPO-ANCA-associated vasculitis] Study Group) performed a prospective, open-label, multi-center trial to evaluate the usefulness of severity-based treatment in Japanese patients with myeloperoxidase-anti-neutrophil cytoplasmic antibodies (MPO-ANCA)-associated vasculitis. Patients with MPO-ANCA-associated vasculitis received a severity-based regimen according to the appropriate protocol: low-dose corticosteroid and, if necessary, cyclophosphamide or azathioprine in patients with mild form; high-dose corticosteroid and cyclophosphamide in those with severe form; and the severe-form regimen plus plasmapheresis in those with the most severe form. We followed up the patients for 18 months. The primary end points were the induction of remission, death, and end-stage renal disease (ESRD). Fifty-two patients were registered, and 48 patients were enrolled in this study (mild form, n = 23; severe form, n = 23; most severe form, n = 2). Among the 47 patients who received the predefined therapies, 42 achieved remission within 6 months, 5 died, and 1 developed ESRD. Disease flared up in 8 of the 42 patients with remission during the 18-month follow-up period. The JMAAV trial is the first prospective trial for MPO-ANCA-associated vasculitis to be performed in Japan. The remission and death rates were comparable to those in several previous clinical trials performed in western counties. The regimen employed in this trial was tailor-made based on patients' disease severity and disease type, and it seems that standardization can be consistent with treatment choices made according to severity.