Previous reports have suggested a relationship between changes in indirect immunofluorescence antineutrophil cytoplasm autoantibody (ANCA) titres and disease activity in patients with Wegener's granulomatosis (WG). We analyzed retrospectively the data from 37 patients with biopsy-proven WG during a median follow-up period of 34 (range 8-60) months, during which 532 serial ANCA measurements had been made. A fourfold increase in ANCA titre and/or a positive titre following a series of negative results occurred on 82 occasions, only 19 (23%) associated with clinical relapse, 25 (31%) with intercurrent infection, and 32 (38%) when patients were considered clinically well. The sensitivity of a significant increase in ANCA titre associated with clinical relapse was 43%, with a positive predictive accuracy of 23%. The combination of an increased C-reactive protein-plasma viscosity and a significant increase in ANCA titre occurred on 37 occasions, 15 (40%) with relapse and 17 (46%) with infection. The results of this study suggest that patients with WG who have significant increases in ANCA titres should be carefully monitored. Increased immunosuppression based solely on increased ANCA titres is not justified, as if this had been practiced in this study, then a large number of patients who had infective episodes would have received additional immunosuppression from which they may not have benefited.