In systemic vasculitis reliable detection of myeloperoxidase antibodies (MPO-Abs) is of great clinical importance in the diagnosis and follow-up of patients. We have studied whether circulating myeloperoxidase (MPO) could have an effect on MPO-Ab findings. Serum MPO and MPO-Abs were measured in 50 healthy individuals, 35 patients and in the follow-up samples from two patients with Wegener's granulomatosis. Heating the sera at 56 degrees C for 30 min reduced the concentration of immunoreactive MPO both in control and patient sera. In 71% of the patient sera heating made initially negative MPO-Abs detectable. In a few cases with severe vasculitis the antibody findings remained totally negative. These results, together with the data from the follow-up samples from two patients with Wegener's granulomatosis, revealed that the serological diagnosis of vasculitis may be considerably delayed if only native samples are analysed for MPO-Abs. These findings are of considerable clinical significance for the interpretation of MPO-Ab results. Circulating myeloperoxidase affects MPO-Ab measurements, causing false negative findings in MPO-Ab assays. Therefore, it is recommended to denaturate circulating MPO by heating the sera before the analysis of MPO-Abs and to re-evaluate the cut off-values.