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Objective: To examine concentrations of circulating antibodies targeting C3a and C5a complement receptors in antineutrophil cytoplasmic autoantibody (ANCA)–associated vasculitis (AAV) and analyze their association with disease activity. Methods: Concentrations of antibodies against C3a and C5a complement receptors (anti‐C3aR and anti‐C5aR) and plasma complement fragments C3a and C5a were determined in patients with AAV (n = 110; granulomatosis with polyangiitis [GPA; n = 82] or microscopic polyangiitis [MPA; n = 28]), systemic lupus erythematosus (SLE) patients as disease controls (n = 36), and healthy donors (n = 220). C3aR and C5aR expression by circulating neutrophils, monocytes, and T cells was analyzed using flow cytometry. Clinical data were assessed at time of serum sampling and during follow‐up for 60 months. Results: In AAV, anti‐C3aR and anti‐C5aR antibodies were decreased (P = 0.0026 and P ≤ 0.0001, respectively). In remission, anti‐C3aR antibody concentrations rose to values comparable to healthy donors, whereas anti‐C5aR antibody concentrations did not. In GPA, anti‐C5a and anti‐C5aR antibody concentrations inversely correlated with each other (r = −0.6831, P = 0.0127). In newly diagnosed GPA, decreased concentrations of anti‐C5aR antibodies but not anti‐C3aR antibodies were associated with disease activity (P = 0.0009). Moreover, low anti‐C5aR antibodies were associated with relapse in GPA (hazard ratio 3.54, P = 0.0009) and MPA (hazard ratio 4.41, P = 0.0041). The frequency of C5aR‐expressing cells within T cell populations was increased in GPA (P = 0.0021 for CD4+ T cells; P = 0.0118 for CD8+ T cells), but not in MPA. Conclusion: Low concentrations of anti‐C5aR antibodies reflect disease activity and are associated with an increased risk for relapse in AAV. [ABSTRACT FROM AUTHOR]