Eosinophils may infiltrate the lung tissue, thus impairing gas exchange and causing several symptoms as dyspnea, fever, and cough. This process may be secondary to several factors, including drugs or parasite migration, or primary (idiopathic). Acute eosinophilic pneumonia is life-threatening and presents frequently in young smokers as an acute hypoxemic respiratory failure of generally less than a week with bilateral lung infiltrates, frequently misdiagnosed as severe community-acquired pneumonia. This patients present without peripheral eosinophilia but usually have more than 25% eosinophils on bronchoalveolar fluid. Chronic eosinophilic pneumonia is a protracted disease of usually more than a month before presentation, with a predilection for middle aged asthmatic patients. Hypoxemia is mild–moderate, and there are usually more than 1,000 eosinophils/mm3 of peripheral blood. Bronchoalveolar fluid has high eosinophil levels (usually more than 25%). Migratory peripheral infiltrates are seen in the chest x-ray film. Both acute and chronic eosinophilic pneumonia are treated by glucocorticoids and respiratory support as well as avoidance of any recognized trigger.