An 80-year-old woman positive for myeloperoxidase-antineutrophil cytoplasmic antibody (MPO-ANCA) was admitted with a 3-month history of fever, general malaise, and weight loss, after unsuccessful treatment with antibiotics. Upon admission, her fever persisted, and there was concomitant deterioration of renal function without active urine sediments. Furthermore, she developed hemoptysis, and chest computed tomography (CT) scan revealed bilateral diffuse alveolar hemorrhage. Although a renal biopsy was not performed because of her dementia, we initially suspected microscopic polyangiitis (MPA) on the basis of her clinical course. Because of her poor general condition, she was administered a low dose of prednisolone. Although her fever subsided, she suffered from intractable alveolar hemorrhage and eventually died from respiratory failure. During the autopsy, fibrinoid necrosis was restricted to medium-sized arteries, including the arcuate arteries of the kidneys and the bronchial arteries, without necrotizing crescentic glomerulonephritis and alveolar capillaritis. Therefore, polyarteritis nodosa (PAN) was diagnosed. It is important to distinguish between MPA and PAN because they can lead to life-threatening complications, and their treatment strategies and prognosis are different. When a patient presents with MPO-ANCA, alveolar hemorrhage, and acute renal failure with little evidence of glomerulonephritis, a differential diagnosis of PAN should be made; however, it is difficult to do so without pathological findings. Therefore, pathological examination should be carried out whenever possible.