The study has been designed as a 12-month, open-label, randomized, controlled study comparing the use of prednisone mono-therapy and prednisone and leflunomide combination therapy in treating patients with IgG4-related disease.

The aim of this clinical trial is to determine the safety and efficacy of Leflunomide in treating patients with IgG4-related disease by comparing the outcomes of prednisone and leflunomide combination therapy with prednisone mono-therapy. The follow-up period will be 12 months. During the follow-up period, results of laboratory tests and image examinations, IgG4-RD RI and other parameters which can reflect treatment response as well as adverse effect events will be recorded.

Inclusion Criteria:

1. Age between 18 and 80 years.
2. Diagnosis of IgG4-RD according to either Consensus statement on the pathology of IgG4-related disease (for those who have undergone biopsies) or 2011 Comprehensive diagnostic criteria for IgG4-related disease. Both of the two criteria for diagnosis are specified below.

(1)Consensus statement on the pathology of IgG4-related disease

1. Histopathologic features consisting of dense lymphoplasmacytic infiltrate, fibrosis(usually storiform in character)and/or obliterative phlebitis within involved organs.
2. Either an elevated IgG4+/IgG+cell ratio of >40% within the affected organs or elevated IgG4-bearing plasma cells per high-power field is necessary. The cut-off number of IgG4-bearing plasma cells per high-power field is different depending upon the types of affected organs and specimens(through surgery or needle puncture biopsy).

(2)2011 Comprehensive diagnostic criteria for IgG4-related disease

1. Clinical examination showing characteristic diffuse/localized swelling or masses in single or multiple organs.

2. Hematological examination shows elevated serum IgG4 concentrations(135 mg/dl).

3. Histopathologic examination shows marked lymphocyte and plasmacyte infiltration and fibrosis or Infiltration of IgG4+ plasma cells(ratio of IgG4+/IgG+ cells > 40% and >10 IgG4+ plasma cells/HPF).

   Definite: a + b + c,Probable: a + c,Possible: a + b

4. Excluded from malignant tumors of each organ (e.g. cancer, lymphoma) and similar diseases (e.g. Sjögren's syndrome, primary sclerosing cholangitis, Castleman's disease, secondary retroperitoneal fibrosis, Wegener's granulomatosis, sarcoidosis, Churg-Strauss syndrome) by additional histopathological examination.

5. Even when patients cannot be diagnosed using the Comprehensive diagnostic criteria, they may be diagnosed using organ-specific diagnostic criteria for IgG4-RD, such as Diagnostic criteria for IgG4-Mikulicz's disease.

Exclusion Criteria:

1. Subjects having received steroids or immunosuppressants in recent 3 months will be excluded.
2. Subjects who were hypersensitive to leflunomide will be excluded.
3. ALT and/or AST is more than two folds of the upper limit of relevant reference value at baseline.
4. WBC is less than 3×10*9/L at baseline.
5. Female patients who are pregnant or breastfeeding.
6. Known significant concurrent medical disease.