A 46-year-old man presented with progressive dyspnea of acute onset, nonproductive coughing, and a high fever. He had been in his usual good health until symptoms began on the previous day. The chest roentgenogram revealed Kerley A and B lines, perivascular cuffing, hilar haze, and bilateral pleural effusins. Body temperature was 37 degrees C and PaO2 was 42 Torr. All clinical and radiographic signs improved. On the 12th hospital day, dyspnea and diffuse infiltration shadows on the chest roentgenogram suddenly occurred again. Acute eosinophilic pneumonia was strongly suspected because of increasing peripheral eosinophilia, and from the results of histologic examination of a specimen obtained by transbronchial lung biopsy. Without steroid treatment, the patient's condition gradually improved. Most cases of eosinophilic pneumonia have been diagnosed as pulmonary infiltration with eosinophilia (PIE). However, this patient did not have PIE syndrome, but instead was given a diagnosis of acute eosinophilic pneumonia, which was first described in 1990. This case may help establish criteria for the diagnosis and steroid treatment of acute eosinophilic pneumonia.