We describe an acute eosinophilic pneumonia (AEP) patient with bronchoalveolar lavage fluid (BALF). Eosinophil cell number (47%), content of interleukin (IL)-5 (8.22 x 10(2) pg/ml) and eosinophil cationic protein (9.25 micrograms/ml) were high in BALF. No eosinophilia was seen in peripheral blood on admission; however, content of IL-5 was 9.47 x 10(2) pg/ml. After methylprednisolone pulse therapy, he improved rapidly with a reduction in eosinophil cell number (7%) and the content of IL-5 (< 100 pg/ml) in BALF. However, a high content of IL-5 (6.9 x 10(2) pg/ml) and transient eosinophilia (17.5%) were seen in peripheral blood. It is important to distinguish between AEP and infectious pneumonia, because of the differing treatments. If the diagnosis of AEP is doubtful, BALF should be performed early.