A 19-year-old man presented with an acute febrile illness and progressive dyspnea. He had begun smoking two weeks before admission. A chest X-ray film revealed Kerley B lines and diffuse infiltration in both lungs. Analysis of bronchoalveolar lavage fluid showed 21% eosinophils. The patient had no history of hypersensitivity to drugs, nor was there any evidence of infectious disease. Acute eosinophilic pneumonia was diagnosed, and his condition improved without steroid treatment. A smoking challenge test was performed. After the test, the patient's body temperature rose to 38.0 degrees C, computed tomograms of the chest showed increased density, and elevated eosinophil levels were again detected in bronchoalveolar lavage fluid. These findings supported the view that beginning to smoke can be a cause of acute eosinophilic pneumonia.