This study is a prospective, open-labelled, randomized, controlled,multi-center clincial trial. The aim of this study is to verify that the remission rate of patients treated with Leflunomide is not inferior to that of patients treated with Azathioprine.

Background The basic theme of AAV is relapse and remission. The maintenance therapy of AAV aimed to reduce or prevent relapse is very challenge. Although many medications have been used for the maintenance of AAV, Leflunomide (LEF) has not studied thoroughly yet. So far, only one study tested the efficacy of LEF in the maintenance therapy for AAV. However, the sample size of this study is small, so large size clinical study is needed to clarify the role of LEF in the maintenance of AAV.

LEF is one of the most frequently prescribed DMARDs in the treatment of rheumatic diseases in China. It is cheap and widely available. Many experiences have been accumulated about its efficacy and safety in Chinese patients with rheumatic diseases. But there is no study to show its effectiveness in the reduction of the relapse of AAV in China. In this study, we try to compare the effectiveness of LEF and AZA, the gold standard for maintenance therapy, in the maintenance of AAV.

Objectives To verify that the effectiveness of LEF in reducing relapse is not inferior to AZA by comparing the relapse rate of LEF and AZA during the 18 month maintenance treatment of AAV.

Study Design This is a prospective, randomized, open-label, control, non-inferiority study.

Inclusion Criteria:

1. Patients age 18 to 75 years, both genders can be included.
2. Patients who are newly diagnosed or relapsing granulomatosis with polyangiitis, microscopic polyangiitis, or EGPA in complete remission after combined treatment with glucocorticoids and pulse cyclophosphamide or Rituximab. Remission is defined as a Birmingham Vasculitis Activity Score(BVAS version 3) of 0.
3. Patients must fulfill the 1990 ACR classification criteria of GPA, EGPA and 2012 modified Chapel Hill classification criteria of MPA.
4. Patients have to be ANCA-positive at diagnosis or during the course of their disease.
5. Patients must sign the informed consent.

Exclusion Criteria:

1. Patients with TPMT gene mutation；
2. Patients who had been treated with either AZA or LEF but relapsed in the past;
3. Patients who had been treated with either AZA or LEF but had to stop due to adverse events or intolerance;
4. Patients who have planned for pregnancy in next 2 years;
5. Patients with severe liver dysfunction(defined as the elevation of liver enzyme 3 times the upper limit or Child grade III) or ESRD；
6. Patients with uncontrolled sever hypertension, diabetes, active bacteria or fungal infection;
7. Patients with active hepatitis virus infection as well as patients who have active mycobacteria infection;
8. Patients who are not eligible according to the judge of the principal investigators or site investigators.

only patient clincial information coud be released to public