We assessed the use of a standardized antineutrophil cytoplasm antibody (ANCA) test in diagnosing Wegener's granulomatosis (WG), microscopic polyarteritis (mPA) and systemic vasculitis (SV). All samples (n = 779) tested for ANCA at our laboratory were identified, and clinical information was obtained for 783/779 patients by questionnaire, and by visits where necessary. The combined prevalence of WG/mPA/SV was 123/738 (17%). The ANCA test was positive in 48/68 WG patients (71%; 38 cANCA, 10 pANCA), 22/43 mPA patients (51%; 12 cANCA, 10 pANCA) and 3/12 SV patients (25%). WG and mPA patients in remission had similar frequencies of positive ANCA to those with active disease. The sensitivity and specificity for WG (71% and 80%) and mPA (51% and 80%) were lower than previously reported. In this high-prevalence population, the overall (WG/mPA/SV) positive predictive value was only 40%, and the sensitivity 59%. Only 29% of positive tests were from patients with active disease. Overall, 78% of test results gave a 'true' prediction. On this basis, a diagnosis of necrotizing vasculitis (WG/mPA/SV) can be neither made nor refuted by ANCA test alone.