Alfa-1-antitrypsin (a-1-AT), C-reactive protein (CRP) and interleukin-6 (IL-6) serum levels were measured during different phases of anti-neutrophil cytoplasmic antibody (ANCA)-positive vasculitis. Correlation between ANCA and these variables was observed. Both a-1-AT and CRP fluctuations indicated changes in activity that could not always be detected by ANCA. On the other hand, these acute phase proteins were also less specific. IL-6 was not as sensitive as CRP and a-1-AT, a clearly increased mean value was observed only in patients with high ANCA titers.