Out of 21 autopsy cases of microscopic polyarteritis (MPA), six cases that died of acute respiratory failure are described. All of these six cases exhibited pauci-immune necrotizing glomerulonephritis. Of these six cases, systemic vasculitis and respiratory failure occurred almost simultaneously in two, whereas respiratory failure developed when vasculitis recurred in the other four. Pathologically, pulmonary changes were diffuse and almost uniform in each case. Interstitial inflammatory cells (alveolitis), hyaline membrane, restructuring, and fibrosis were observed. The degrees of these changes differed from case to case; four cases showed predominantly exudative changes and two cases exhibited proliferative or organizing ones. Clinical and pathological features were consistent with acute interstitial pneumonia (AIP). Immunofluorescent and ultrastructural studies did not suggest deposition of immune complexes at the lesions of alveolitis. An etiologic agent could not be identified in these cases. Some lung lesions including pulmonary hemorrhage have been described in association with MPA; however, cases such as those presented here have rarely been reported. It was considered that AIP could be one of the pulmonary disorders in MPA. Furthermore, the possible pathogenetic role of antineutrophil cytoplasmic antibodies (ANCA) in the evolution of present lung lesion is discussed. Although ANCA were not examined in the present cases, these antibodies should be investigated in future cases.