Successful maintenance therapy with mycophenolate mofetil (MMF) 2 g/d and low-dose oral corticosteroids (OCS) over a period of 15 mo was given to patients with Wegener's granulomatosis (WG) (n = 9) and microscopic polyangiitis (MPA) (n = 2). All patients had severe generalized disease with pauci-immune necrotizing glomerulonephritis and received standard induction therapy with oral cyclophosphamide and OCS for a mean of 14 wk until remission was achieved. Of 11 patients, only one WG patient relapsed in the 14th month of maintenance therapy. Maintenance therapy with MMF was able to further reduce grumbling disease activity as measured by the Birmingham vasculitis activity score (BVAS2) and proteinuria that were still present by the end of induction therapy. OCS could be reduced to a median daily dose of 5 mg and discontinued in three patients. Possible drug-related adverse effects were transient and included abdominal pain, respiratory infection, diarrhea, leukopenia, and a cytomegalovirus-colitis in one patient that was successfully treated with ganciclovir. It is concluded that MMF in combination with low-dose OCS is well tolerated and effective for maintenance therapy of WG and MPA. Long-term treatment with MMF in these diseases is attractive because of its low toxicity. MMF will have to be studied further and compared with cyclophosphamide or azathioprine maintenance therapy in randomized trials.