An 84-year-old man was referred to our hospital because of fever, cough, and hemoptysis. The patient had acute respiratory failure (PaO2 < 40 mmHg) on admission, with diffuse interstitial infiltration and bilateral pleural effusion. The bronchoalveolar lavage fluid was bloody, and contained a high percentage of eosinophils (32%). A diagnosis of acute eosinophilic pneumonia was established, and the patient made a rapid recovery after corticosteroids were administered. When the DLST (drug lymphocyte stimulation test) was performed after the corticosteroid therapy was stopped, it was positive for serrapeptase, which had been prescribed for chronic cystitis for 3 months before the onset of the pneumonia. This was a case of drug (serrapeptase)-induced pneumonitis manifesting as acute eosinophilic pneumonia.