A 66-year-old man came to our hospital with chief complaints of chronic cough and dyspnea in December 2002. Chest CT revealed peripheral and dorsal reticular shadows in the bilateral lung. Although no angiitis was observed in lung specimen obtained from surgical lung biopsy, myeloperoxidase antineutrophil cytoplasmic antibody (MPO-ANCA) results were very high (348 EU/L), and MPO-ANCA-related interstitial pneumonia was diagnosed. With oral prednisolone and cyclosporine, radiographic findings and respiratory function tests were improved, and MPO-ANCA became negative. However, renal disorder resulting from the side effects of cyclosporine developed in September 2007. Although it was improved with cessation of cyclosporine, MPO-ANCA became positive in October 2007 and increased gradually with no new symptoms. In January 2008, the patient developed nasal hemorrhage, hemosputum, fever, and cough. Despite intensive therapy, he died, and the autopsy revealed alveolar hemorrhage with necrotizing vasulitis, leading to a diagnosis of microscopic polyangiitis (MPA). This case indicates the importance of strengthening immunosuppressive therapy when a seroconversion of MPO-ANCA occurs after changing therapy, even though long remission has been achieved without any emerging symptoms.