In recent years, the prognosis of Wegener's granulomatosis (WG) has been dramatically improved by treatment with cyclophosphamide and glucocorticoids, if the diagnosis is made correctly in the early stage of the disease. However, it is often difficult to diagnose of WG relying only on clinical symptoms and histological confirmation with biopsy specimens. Anti-neutrophil cytoplasmic antibody (ANCA) has already been reported to have high sensitivity and specificity for WG. Since February, 1991, we examined this antibody level in the sera of patients suspected of having WG using an indirect immunofluorescence technique, and have experienced nine WG patients who were ANCA positive. Six of the 9 patients showed limited forms of WG before active generalized symptoms developed, and only 3 were diagnosed as having WG with pathologic findings from their biopsy samples. These results show that correct diagnosis of WG is possible with the aid of ANCA determination, even if the patient has a limited form of the disease and even without evidence of characteristic pathologic features of WG on biopsy specimens. Therefore, it is apparent that ANCA determination is a valuable tool for diagnosing WG as early as possible. In Japan, the ANCA test is not as yet a common laboratory examination and has only rarely been used as seromarker for the diagnosis and treatment of WG. We believe that ANCA determination is essential for the diagnosis of WG, especially in its early stage, which leads to a better prognosis for WG patients.