Microscopic polyangiitis (MP) is a primary systemic vasculitis predominantly affecting small
blood vessels. Following the widespread introduction of ANCA testing, the primary systemic
vasculitis (SV), Wegener?s granulomatosis (WG) and microscopic polyangiitis (MP) appear to be
more frequent than was previously thought (see definitions in Appendix 6). In addition, the
existence of early and organ-limited forms of these diseases, such as renal-limited
vasculitis (RLV) is now clearly recognized. Their annual incidence exceeds 20 per million per
year and they account for at least 5 % of the causes of end stage renal failure. The two
diseases share many features of their histology, serology and response to treatment, pointing
to similarities in their pathogenesis, which have justified a common approach to their
management. The standard treatment with corticosteroids (CS) and cyclophosphamide (CYC) is
usually effective at controlling active disease but continued treatment is necessary to
prevent disease relapse. Due to the cumulative toxicity associated with CYC treatment,
alternatives have been looked for. Mycophenolate mofetil (MMF) has been used to treat
patients with a variety of immune-mediated nephritides, including ANCA-associated vasculitis,
with less toxicity than CYC but with variable outcome. The present trial will examine whether
substitution of oral CYC with oral MMF is equally efficient for induction of remission with
less adverse effects in cases of MP with mild to moderate renal involvement. All patients
will receive the same regimen of oral prednisone + MMF. Prednisone will be tapered to a stop
after 24 weeks but MMF will continue for a total of 18 months unless there is worsening or
persistent disease. The trial ends after 18 months.