A 19-year-old female was admitted to our hospital because of a sudden onset fever and cough, and she was diagnosed to have acute eosinophilic pneumonia (AEP). The cause was thought to be cigarette smoking, because she had started smoking just before the development of AEP and her condition improved after cigarette smoking cessation, without corticosteroid treatment. The cytokines which are thought to be involved in eosinophilic accumulation in the lungs were analyzed using bronchoalveolar lavage fluid (BALF) and serum. Of the analyzed cytokines, only regulated on activation, normal T cell expressed and secreted (RANTES) increased in the serum after the improvement. RANTES is a unique chemokine which attracts not only eosinophils, but also T cells. Interestingly, in this case, the eosinophil count in the blood increased in parallel with the lymphocyte count after the improvement. These findings are interesting because it may help to understand the pathogenesis of AEP and the role of RANTES.