OBJECTIVE:. The predictive value of antineutrophil cytoplasmic antibodies (ANCA) as markers of clinical activity of small vessel vasculitis is controversial. We reviewed the outcome of patients with ANCA-associated vasculitis from a single center and evaluated the predictive value of ANCA as markers of relapse. METHODS: The medical history of all consecutive patients with ANCA-associated vasculitis followed at our outpatient clinic was retrospectively reviewed. ANCA were monitored by immunofluorescence and by ELISA (antiproteinase 3 and antimyeloperoxidase). RESULTS: Twenty-three patients with Wegener's granulomatosis and 13 with microscopic polyangiitis were followed for a median period of 5 years (1 mo-16 yrs). Most patients were treated with combined corticosteroids and cyclophosphamide. In the Wegener's granulomatosis group, remission was obtained in 21 of 23 patients (91%), within 4-48 weeks (median 7.5); relapses occurred in 74%. In the microscopic polyangiitis group, remission was obtained in all patients within 3-38 weeks (median 8); relapses occurred in 33%. In contrast to persistently (> 6 mo) elevated ANCA titers, which were not significantly associated with disease relapse, the predictive value of an acute rise in ANCA titers was strongly associated with the magnitude of the increase. CONCLUSION: Our study demonstrates the weak predictive value of persistently elevated ANCA titers, in contrast to acute rises in ANCA titers. Although an acute rise in ANCA titers may help in a decision whether to introduce immunosuppressive therapy, the final decision has to be based on both clinical and laboratory markers.