Studies in animal models suggest that complement activation is crucial in the pathogenesis of anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV). Here we investigate the circulating complement activation profile of 66 patients with active stage AAV compared to that of 54 patients with AAV in remission. Plasma levels of C3a, C5a, soluble C5b-9, and Bb, all determined by enzyme-linked immunosorbent assay, were significantly higher in active stage than in remission of AAV, while plasma levels of properdin were significantly lower in the former than the latter disease stage. There was no significant difference in the plasma levels of C4d between active stage and remission. The plasma level of Bb in patients with active AAV significantly correlated with the proportion of total and cellular crescents in the renal biopsy, the erythrocyte sedimentation rate, and the Birmingham Vasculitis Activity Scores. Thus, systemic activation of complement by the alternative pathway takes place in human AAV. Circulating Bb might be a useful biomarker in assessing disease activity of AAV.