A 48-year old man was admitted with idiopathic fever, and subsequently diagnosed as having hemophagocytic lymphohistiocytosis (HLH). Though an extensive series of laboratory examinations failed to demonstrate an apparent etiology, empirical use of steroids achieved remission. About two years later, the patient developed Pneumocystis Jiroveci pneumonia and was diagnosed as HIV-positive. Based on this case, HIV-screening tests would be performed whenever we encounter HLH in Japan, where the number of HIV-positive patients is increasing.