Acute eosinophilic pneumonia is a sudden and severe febrile illness that can result in life-threatening respiratory failure. Because this disease can mimic other diseases causing acute respiratory failure, the diagnosis may be missed or delayed. It is clinically distinguishable from other pulmonary eosinophilic diseases and can be diagnosed with bronchoalveolar lavage. Treatment with corticosteroids results in rapid reversal of respiratory failure and complete recovery, generally without relapse. In some patients, acute eosinophilic pneumonia may be caused by drugs, tobacco smoke, or other inhaled substances; however, in the majority of patients, the condition is idiopathic.