The purpose of this study is to determine the efficacy and safety of a new drug, mycophenolate mofetil, for the treatment of relapses of ANCA-associated vasculitis (Wegener's granulomatosis or microscopic polyangiitis). Therefore, we compare the standard therapy with cyclophosphamide to mycophenolate mofetil.

The investigators expect mycophenolate mofetil to be less toxic and almost equally effective as cyclophosphamide.

Treatment of ANCA-associated vasculitis consists of two phases: remission induction with highly effective, but also relatively toxic drugs, and, secondly, after remission is achieved, maintenance therapy with less toxic drugs. The standard induction therapy of a relapse of Wegener's granulomatosis or microscopic polyangiitis consists of the combination of cyclophosphamide and prednisolone. Although this induction therapy is very effective, it is very toxic as well.

Searching for an alternative for cyclophosphamide, we will test the efficacy and safety of a new combination therapy with mycophenolate mofetil and prednisolone. We will compare the effect and safety of the standard induction therapy with the new therapy. When relapses occur, patients will be randomized for either the standard therapy with cyclophosphamide or for mycophenolate mofetil.

Inclusion Criteria:

• First or second relapse ANCA-associated vasculitis
• PR3- or MPO-ANCA antibodies present or histological proof of relapse
• Adult

Exclusion Criteria:

• Severe alveolar bleeding or (imminent) respiratory failure
• Renal failure (serum creatinine >500 umol/L or dialysis)
• Maintenance therapy before start of study consisting of: cyclophosphamide > 100 mg/day or prednisolone >25 mg/day
• Intolerance or allergy for cyclophosphamide, mycophenolate mofetil or azathioprine
• Gravidity or inadequate anticonception