We investigated circulating interleukin-18 concentrations in patients with ANCA-associated vasculitis (ASV) and healthy control subjects, and included a group of hemodialysis patients, a patient group previously reported to show high IL-18 plasma levels. Anti-proteinase 3 (PR3) and anti-myeloperoxidase (MPO) serum levels were also measured. Interestingly we found significantly increased serum IL-18 concentrations in ASV patients as compared to healthy controls, 437 vs. 185 pg/ml (p < 0.0001). The increase of IL-18 production was similar irrespective of presence of autoantibodies to PR3 or MPO. As expected the hemodialysis patients also showed significantly increased circulating IL-18 concentrations as compared to control subjects.