OBJECTIVE : Rituximab was proven superior to azathioprine for maintenance treatment of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). The high cost of rituximab might, however, limit its routine use. This study determined the cost-effectiveness of intravenous rituximab (5 x 500 mg until month 18), versus oral azathioprine (2 mg/kg per day, gradually decreased between month 12 and 22), for maintenance treatment of patients with granulomatosis with polyangiitis, microscopic polyangiitis, or renal-limited vasculitis, aged 18-75. METHODS : We performed a single-trial based economic evaluation. MAINRITSAN was a 28-month multicentre, prospective, randomised, controlled open-label trial. We estimated the cost of healthcare resources and quality of life using prospectively collected data. Healthcare costs were estimated from the perspective of the French Social Health Insurance's perspective, using 2016 tariffs for reimbursement. Utilities were derived from Short Form 36 scores. We estimated total average cost, incremental cost per incremental relapse averted and per quality-adjusted life-year (QALY) gained. Sensitivity analyses were performed to assess uncertainty over relapses, severe adverse events, discount rate, utility weights, time horizon and the cost of rituximab. Costs drivers were tested using a generalised linear model. RESULTS : Total average costs were €13,387 (€11,605-€15,646) and €10,217 (€7,567-12,949) in the rituximab and azathioprine groups respectively. The incremental cost-effectiveness ratio (ICER) was €12,824 per relapse averted and the incremental cost-utility ratio (ICUR) €37,782 per QALY gained. Besides the unit cost of rituximab, the major cost drivers were relapses and severe adverse events. CONCLUSIONS : Maintenance treatment by rituximab could be cost-effective for preventing relapses in patients with AAV.