Rises in antineutrophil cytoplasmic antibodies (ANCA) directed against proteinase 3 (PR3) have predictive potential for a relapse of Wegener's granulomatosis (WG). To assess the value of image analysis for monitoring ANCA levels, we measured PR3-ANCA in a cohort of PR3-ANCA positive patients with WG that were prospectively followed in our clinic and compared findings with other techniques. ANCA levels were measured serially by quantitative image analysis, titration in indirect immunofluorescence (IIF), two different directly coated antigen-specific enzyme-linked immunosorbent assays (ELISA), and a capture ELISA using a PR3-specific monoclonal antibody in 16 consecutive WG patients prior to a renal relapse, and in 16 age- and sex-matched patients with inactive WG. The positive predictive value (PPV) of an increase in ANCA titers by image analysis for relapse was 69% (11 of 16). The PPV of an increase in ANCA was 61% (11 of 18) by IIF, 71% (12 of 17) by a commercial direct ELISA, 63% (12 of 19) by in-house direct ELISA, and 75% (12 of 16) by capture ELISA. The negative predictive value (NPV) of the absence of an increase in ANCA titers by image analysis for relapse was 69% (11 of 16). The NPV of the absence of an increase in ANCA was 64% (9 of 14) by IIF, 73% (11 of 15) by a commercial direct ELISA, 63% (9 of 13) by in-house direct ELISA, and 75% (12 of 16) by capture ELISA. In conclusions, quantitative image analysis is a novel technique based on the principle of IIF to quantify ANCA levels in a single dilution in a patient sample. No major differences were observed between image analysis and the other techniques in their capacity to predict relapses of disease activity.