Idiopathic acute eosinophilic pneumonia (IAEP) is characterized by the rapid accumulation of eosinophils in the lungs. Eosinophils are a type of white blood cell and are part of the immune system. IAEP can occur at any age but most commonly affects otherwise healthy individuals between 20 and 40 years of age. Signs and symptoms may include fever, cough, fatigue, difficulty breathing (dyspnea), muscle pain, and chest pain. IAEP can progress rapidly to acute respiratory failure. The term “idiopathic” means the exact cause for the overproduction of eosinophils is not known. Possible triggers of acute eosinophilic pneumonia include cigarette smoking, occupational exposure to dust and smoke, and certain medications. Diagnosis of IAEP generally involves a bronchoscopy and bronchoalveolar lavage (BAL). Treatment with corticosteroids is effective in most cases. Because IAEP often progresses rapidly, respiratory failure can occur; in these cases, mechanical ventilation is required.