One hundred and six patients with Wegener's granulomatosis (WG) were studied for the presence of antineutrophil cytoplasmic antibodies (ANCA). In 53 patients serial ANCA determinations were obtained. C-ANCA positivity was a sensitive (88%) marker of active WG. However, changes in serial titers were temporally concordant with a change in disease status in only 55% of patients. Furthermore, a rise in c-ANCA titer preceded clinical exacerbation of disease in only 24% of patients who had been in remission or had low grade, smoldering disease. A rise in c-ANCA titer alone should not be considered a priori evidence of impending relapse, and does not justify modification of immunosuppressive therapy.