Titration of antibodies against neutrophil cytoplasmic antigens (cANCA), as detected by indirect immunofluorescence, is shown to be clinically useful for monitoring disease activity in Wegener's granulomatosis and microscopic polyarteritis. Ten patients were followed (eight from presentation) prospectively for up to 2 years; during this time there were six episodes of vasculitic relapse in four patients and five infective episodes and one pulmonary embolus in four patients. Titres of cANCA were markedly raised, both at presentation (1/32-1/2048) and at vasculitic relapse (1/125-1/1048) but not in infection or embolism (negative, 1/16). Thus the titre of these antibodies can distinguish nonvasculitic illness from vasculitic relapse, in contrast to C-reactive protein levels which were raised in both. Titres of cANCA fell gradually after vasculitic relapse, in keeping with the half-life of IgG (3 weeks). C-reactive protein is a better measure of recovery.