Fussner et al. used data from 93 patients with anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) from the RAVE trial who had ANCAs targeting proteinase 3 (PR3-ANCAs) to investigate the association between PR3-ANCA levels and the likelihood of AAV relapse. In the 55 patients in whom the disease relapsed, rises in PR3-ANCA levels were observed by direct ELISA in 25 patients and by capture ELISA in 21 patients. The rises in PR3-ANCA levels detected by direct ELISA were significantly associated with severe relapse, especially in patients with renal involvement and alveolar haemorrhage. An increased risk of relapse associated with rises in PR3-ANCA levels was identified in patients treated with rituximab, but not in those treated with cyclophosphamide or azathioprine. The findings show that the association between PR3-ANCA levels and risk of relapse is affected by disease phenotype and type of treatment, and also by the way PR3-ANCA levels are measured.