ANCA-tests are not always useful in evaluation of disease activity of patients with Wegener's granulomatosis (WG) and microscopic polyarteritis(MP). If ANCA-tests are failing in patients with Wegener's granulomatosis or microscopic polyarteritis, markers of activated coagulation are helpful in evaluation of disease activity. The measurement of anticardiolipin antibodies(ACA) may allow to discriminate between MP and WG, since ACA are only significantly elevated in patients with MP. Anticardiolipin antibodies (ACA) are acquired autoantibodies that are found in patients with a wide spectrum of clinical conditions, mainly systemic lupus erythematosus and related disorders. They are associated with increased thrombotic tendency. We investigated in patients with ANCA-positive vasculitis whether membrane phospholipids exposed in injured endothelial cells might stimulate the formation of ACA, thus contributing to the vascular thrombotic occlusion besides the activation of coagulation.