Acute and chronic eosinophilic pneumonia can be distinguished by their clinical, laboratory, and radiographic features. Patients with both acute and chronic eosinophilic pneumonia present with cough, dyspnea, and fever. Patients with chronic eosinophilic pneumonia present subacutely over weeks to months but patients with acute eosinophilic pneumonia present within 5 days of symptom onset. Chest radiographs in chronic eosinophilic pneumonia show peripheral alveolar infiltrates. In contrast, radiographs in acute eosinophilic pneumonia show mixed interstitial and alveolar infiltrates, Kerley B lines, and pleural effusions. Both disorders are characterized by high percentages of bronchoalveolar lavage eosinophils, but high numbers of blood eosinophils accompanies only chronic eosinophilic pneumonia. The diagnosis of both disorders can usually be made based on clinical and radiographic findings; however, lung biopsy is occasionally necessary to distinguish the eosinophilic pneumonias from other eosinophilic lung diseases. In both conditions, patients will respond rapidly and completely to corticosteroids but patients with chronic eosinophilic pneumonia usually relapse if less than 6 months of treatment is given, whereas patients with acute eosinophilic pneumonia do not relapse after a brief course of treatment.