Background: Mycophenolate mofetil (MMF) is an immune suppressive initially introduced for the prevention of solid organ allograft rejection that is increasingly used in autoimmune conditions, including vasculitis. Methods: This retrospective study evaluated the efficacy and tolerability of MMF in 51 sequential patients with anti-neutrophil cytoplasm antibodies-associated systemic vasculitis (AASV) treated in a single centre between 2001 and 2004. Results: The mean age was 54 years and median disease duration was 36 months. A mean of 3.5 systems were involved and the previous median exposure to cyclophosphamide was 9 g. MMF was administered either as remission maintenance therapy (29/51, 56.9%) or as treatment for active disease (22/51, 43.1%). The mean duration of MMF therapy was 20 months and the mean MMF dose during the first year was 1.6 g/day. 14/29 (48.3%) of those receiving MMF for remission maintenance therapy eventually relapsed with a mean time to relapse of 14 months. Of those receiving MMF for relapsing disease, 3 failed to respond to therapy while the rest achieved remission by 3.9 months. However, 9 of these subsequently flared; mean time to disease flare was also 14 months. MMF was withdrawn in 28 patients (54.9%) because of treatment inefficacy in 21, severe adverse events in 5 and intolerance in 2. Of the 51 treated, 36 (70.6%) experienced at least 1 side effect, namely infections in 24, gastrointestinal side effects in 12 and psychological events in 6 patients. Conclusions: We have observed varying efficacy of MMF in AASV, with over 50% of patients with relapsing disease achieving remission and marked falls in concomitant steroid doses. However, longer follow-up indicates a subsequent relapse rate of over 50% that may be associated with low MMF dosing.