 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. 