OBJECTIVE: To contrast the effect of the burden of vasculitis activity with the burden of adverse events on one-year mortality of patients with ANCA-associated vasculitis (AAV). METHODS: We assessed outcome and adverse events in patients prospectively recruited to 4 European AAV clinical trials. We included data on 524 patients with newly diagnosed AAV. The burden of adverse events was quantified using a severity score for leukopenia, infection and other adverse events, with an additional weighting for follow up duration. A "combined burden of events score (CBOE)" was generated for each patient by summing the individual scores. Vasculitis severity was quantified using BVAS and GFR. RESULTS: One-year mortality probability was 11.1%; 59% and 14% of deaths were due to therapy associated adverse events and active vasculitis respectively. Using Cox regression analysis, infection score (p7 was 14.4 (95% CI 8.4 to 24.8). Age and GFR were independent predictors of CBOE score. CONCLUSIONS: The greatest threat to patients with AAV in the first year of therapy is from adverse events rather than active vasculitis. Accumulation of adverse events, monitored using this scoring method, should prompt increased awareness that the patient is at high risk of death.