In the last decade, serological detection of anti-neutrophil cytoplasmic antibodies (ANCA) and of anti-glomerular basement membrane (GBM) antibodies has tremendously facilitated the diagnosis of small vessel vasculitides. Once diagnosed, these diseases have proven to be treatable. However, in contrast to anti-GBM disease, ANCA-associated vasculitides are chronic diseases with a high relapse rate. Since morbidity in ANCA-associated vasculitides is dictated by the frequency and severity of relapses, much health benefit would be achieved if a relapse could be prevented or early treatment started. Increases in ANCA titers and persistently high ANCA levels indicate a high risk of relapse and warrant clinical evaluation of the patient for signs of relapse. This review will focus on the value of ANCA and anti-GBM antibody testing in diagnosis and on the importance of these tests in follow-up of disease.