Acute eosinophilic pneumonia (AEP) has been described as an idiopathic febrile illness with a duration of less than seven days with severe hypoxemia, pulmonary infiltrates, and no history of asthma. It has been reported that AEP is associated with smoking. Although the pathogenesis of smoking induced AEP is being actively studied, there is no direct histological evidence that smoking actually induces AEP. Recently, we encountered a case of AEP that may have been caused by smoking. We performed a cigarette smoking challenge test to verify that smoking was indeed the cause of AEP in this patient. Smoking induced an increase the proportion of eosinophils in the bronchoalveolar lavage fluid without any respiratory symptoms or abnormal radiological findings. This result suggests that smoking was the cause of AEP in this patient.