We have previously demonstrated that a strong correlation exists between systemic vasculitis with proteinase 3-anti-neutrophil cytoplasm antibodies (PR3-ANCA) and heterozygosity for alpha 1-antitrypsin (alpha 1-AT) deficiency, PiZ. In the present study we characterized the PiZ-positive subgroup by laboratory findings, clinical features and outcome. The series studied consisted of 18 PiZ-positive and 81 PiZ-negative PR3-ANCA patients, comparable in sex ratio, age, C-reactive protein concentrations and renal function at diagnosis, and treatment. PiZ-positive patients had a more disseminated disease as reflected by the number of affected organs (P < 0.01). We found no group differences in relapse tendency. Overall mortality was 39% (7 of 18) in the PiZ-positive and 16% (13 of 81) in the non-PiZ group (P = 0.048). When survival analysis was restricted to 66 patients included in the study at disease onset, the group difference was significant (P = 0.016). The results suggest that the subnormal response of plasma alpha 1-AT seen in PiZ-heterozygotes enhances the risk of dissemination of the vasculitic process and the risk of a fatal outcome. We consider alpha 1-AT phenotyping to be justified in cases of PR3-ANCA-positive vasculitis. Treatments decreasing plasma alpha 1-AT (such as plasmapheresis without plasma replacement) may be deleterious.