Eosinophilic lung diseases comprise eosinophilic pneumonia, which may present with chronic or acute onset, or as Löffler syndrome. The diagnosis of eosinophilic pneumonia relies on clinical imaging and the demonstration of alveolar eosinophilia. Lung biopsy is generally not necessary. Peripheral blood eosinophilia is common but may be absent at presentation in idiopathic acute eosinophilic pneumonia, which may be misdiagnosed as severe infectious pneumonia. Extra-thoracic manifestations should raise the suspicion of Churg-Strauss syndrome. All possible causes of eosinophilia (especially fungus infection or drug or toxic exposure) must be thoroughly investigated before the diagnosis of idiopathic disease is made.