The antineutrophil cytoplasmic antibody (ANCA) test has been shown to be important in helping to confirm the diagnosis and following the clinical course of microscopic polyarteritis and Wegener's granulomatosis. So called "false positive" test results have been reported, but usually in patients without any clinical evidence of these diseases, and the "false positive" result ignored. I wish to report 4 cases, in which a diagnosis of microscopic polyarteritis/Wegener's granulomatosis was considered as part of the differential diagnosis based on the clinical findings. The ANCA test was positive for cytoplasmic staining in 2 cases and perinuclear in 2 others. The combination of a positive ANCA result and the clinical possibility of Wegener's granulomatosis and/or microscopic polyarteritis resulted in the prescription of immunosuppressive treatment, with the consequent mortality of one patient and significant morbidity in two of the other cases.