Sera from 105 patients with Wegener's granulomatosis (WG) and 1,500 control subjects (with, among other diseases, primary or secondary vasculitis, collagen disease, glomerulonephritis or various granulomas) were examined with respect to sensitivity, specificity and importance of anticytoplasmic antibodies (ACPA) as markers of disease activity. The antibodies were demonstrated in 77 of the 105 patients (73%) with WG and six of the controls (less than 1%). They were always present in active WG (57 patients). In a prospective study of 21 closely followed patients with WG there was a close correlation between ACPA titre and the disease activity as measured by clinical and biochemical criteria. After induction of remission there was in all instances a clear fall in titre, while the titre rose again on exacerbation but remained unchanged during superinfection. Thus anticytoplasmic antibodies are useful both in primary diagnosis and in estimating disease activity of WG.