OBJECTIVE: To investigate the clinical significance of peptidylarginine deiminase type 4 (PAD4) in the pathogenesis of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). To make a primary observation on the relationship of chronic bronchitis and bronchiectasis (CB) with the pathogenesis of AAV by PAD4. METHODS: The sera from 13 patients with AAV, 13 patients with CB, 11 patients with rheumatoid arthritis (RA), 11 patients with primary chronic kidney disease (CKD) and 12 normal controls were collected. Serum PAD4 was detected using commercial ELISA kits. The serum levels of PAD4 were compared not only among the different groups but also between the activity and remission stage of the same disease. The associations between serum PAD4 and the Birmingham Vasculitis Activity Score (BVAS) of AAV were further investigated. RESULTS: (1) The serum levels of PAD4 in patients with AAV, RA and CB at activity stage were all higher than that in the normal controls (P<0.001, respectively, α'=0.007). The serum level of PAD4 in patients with CB at remission stage and that in CKD group were not found elevated compared with the normal controls (P=0.02, P=0.085, respectively, α'=0.007). (2) At activity stage, among the groups of simple AAV, AAV with a long history of CB and CB without AAV, no significant difference was detected. While at remission stage among the 3 groups, the serum level of PAD4 was at the lowest level in CB group without AAV. (3) The serum level of PAD4 in some patients with CB without AAV were found still higher at remission stage. (4) The serum level of PAD4 in AAV with renal damage at activity stage was positively correlated with BVAS (the activity score of AAV, r=0.71, P=0.02). CONCLUSION: PAD4 is involved in the pathogenesis of AAV. Whether some patients with CB might progress to AAV by the link with PAD4 still need further investigation.