We examined an 86-year-old man with acute respiratory failure. A chest roentgenogram showed diffuse reticular shadows. Transbronchial biopsy revealed thickening of the alveolar septa accompanied by moderate eosinophil infiltration. After admission to the hospital, the patient's symptoms immediately improved without any medication. Clinical course and pathologic findings suggested acute eosinophilic pneumonia caused by a hypersensitivity reaction.