We describe the clinical features, eosinophil surface analysis, and concentration of interleukin-5 (IL-5) in bronchoalveolar lavage (BAL) and in peripheral blood (PB) of a young male patient with acute eosinophilic pneumonia. Eosinophilic pneumonia was diagnosed by BAL and by its clinical course. There was no evidence of an infectious etiology, and the patient showed rapid improvement with methylprednisolone pulse therapy (1 gm/d for 3 d). Significant differences in the expression of surface molecules such as CD54, HLA-DR, and CD69 were observed between BAL and PB eosinophils; IL-5 concentrations in BAL and in PB before treatment were 450 pg/ml and 700 pg/ml, respectively. After treatment, IL-5 concentration in PB was reduced to below 100 pg/ml. The patient has shown no recurrence after treatment.