Broadly speaking, eosinophilic pneumonia is a disease process characterized by an intraalveolar exudate containing large numbers of eosinophils, frequently with associated peripheral eosinophilia. Eosinophilic pneumonia may be the primary pathologic manifestation of a disease, such as acute eosinophilic pneumonia and chronic eosinophilic pneumonia, or may be a secondary pathologic manifestation of another disease process, such as allergic bronchopulmonary aspergillosis, granulomatous infections, Churg-Strauss syndrome, or rheumatoid arthritis. Acute eosinophilic pneumonia generally occurs in patients in the third decade as a sudden and severe febrile illness that may result in life-threatening respiratory failure. Acute eosinophilic pneumonia is associated with greater than 25% eosinophilia on bronchoalveolar lavage. Most cases are idiopathic, but in some patients, it may be caused by infection; drugs, including antibiotics and nonsteroidal anti-inflammatory drugs; or cigarette smoking, among other etiologies. Patients generally exhibit rapid recovery either spontaneously or with corticosteroid therapy. Chronic eosinophilic pneumonia is an uncommon subacute disease that may be confused clinically with bacterial pneumonia or other causes of lung consolidation. Chronic eosinophilic pneumonia is characterized by symptoms—primarily cough and dyspnea, but also fever, weight loss, and night sweats—waxing and waning over a period of months, or even years. Patients frequently improve after corticosteroid therapy but more than 80% relapse upon discontinuation of therapy. The vast majority of patients exhibit peripheral eosinophilia. Both men and women of all ages may be affected, but chronic eosinophilic pneumonia is commonly found in women asthmatics in their fourth and fifth decades. Although the etiology of chronic eosinophilic pneumonia is unknown, studies suggest that an unknown stimulus or insult causes the accumulation of eosinophils in the lung secondary to eosinophilspecific chemoattractants including eotaxin, regulated upon activation, normal T-cell expressed and secreted (RANTES), and to interleukin-5 released from T-helper-2 cells within the lungs.