Acute eosinophilic pneumonia is one of a recently described idiopathic eosinophilic lung disease, whichdiffers from chronic eosinophilic pneumonia. Patients with acute eosinophilic pneumonia develop acute onset ofdyspnea, hypoxemia, diffuse pulmonary infiltrates and pleural effusion on chest radiograph, and show an increasein number of eosinophils in bronchoalveolar lavage fluid or lung biopsy specimen. Prompt and complete response tocorticosteroid therapy without any recurrence is characteristically seen in patient with this disease. Althoughthe etiology of acute eosinophilic pneumonia is not known, it has been suggested to be related to ahypersensitivity phenomenon to an unidentified inhaled antigen. We report four cases of acute eosinophilicpneumonia presented with acute onset of dyspnea, diffuse pulmonary infiltrates on chest radiograph, andeosinophilia in bronchoalveolar lavage fluid in previously healthy adults.