To determine disease specificity and sensitivity of anticytoplasmic autoantibodies (ACPA) for Wegener granulomatosis, as well as their value as a marker of disease activity. Blind analysis of serum samples, retrospective analysis of clinical data on patients, and prospective follow-up of a subgroup of patients with Wegener granulomatosis. The study included 277 patients with Wegener granulomatosis (222 with biopsy-proven disease) and 1657 control patients. University hospital and academic medical center. LABORATORY INVESTIGATIONS: Analysis of 2653 serum samples from 1934 patients for ACPA. Antibody detection was by indirect immunofluorescence and a new type of enzyme-linked immunoadsorbent assay (ELISA). Prospective follow-up was on 172 patients with Wegener granulomatosis. Specificity of ACPA for Wegener granulomatosis measured by indirect immunofluorescence was 99% (CI, 98.9% to 99.7%) and 98% (CI, 97.4% to 99.2%) by ELISA. Sensitivity of ACPA depended on disease activity and extent: It was 67% (CI, 38% to 89%) by immunofluorescence and 60% (CI, 32% to 84%) by ELISA for patients with active locoregional symptomatology (n = 15); and 32% (CI, 14% to 54%) by immunofluorescence and 40% (CI, 21% to 61%) by ELISA for patients in full remission after initial locoregional symptoms (n = 25). The sensitivity was 96% (CI, 89% to 99%) by immunofluorescence and 93% (CI, 86% to 98%) by ELISA for patients with active generalized disease (n = 92). Serial testing was done; every patient with active generalized disease eventually had at least one positive serum sample. Sensitivity decreased to 41% (CI, 22% to 62%) by both immunofluorescence and ELISA for patients in full remission after active generalized disease (n = 27). Levels of ACPA expressed both as immunofluorescence titers and ELISA values (U/mL) correlated well with disease activity. Testing for ACPA in serum of patients with Wegener granulomatosis is valuable for differential diagnosis; furthermore, APCA can be used as a marker to follow disease activity. A new type of ELISA yielded the same results as indirect immunofluorescence for the specificity, sensitivity, and correlation with disease activity of ACPA.