ABSTRACT: A case of infectious mononucleosis (IM) in a previously healthy 43-year-old male is presented. The patient complained of fever and a sore throat of 3 weeks' duration. Although blood tests showed a marked increase in atypical lymphocytes, tests for Epstein-Barr virus (EBV) and cytomegalovirus (CMV) were negative. Human immunodeficiency virus type 1 (HIV-1) antibody was positive on enzymelinked immunosorbent assay (ELISA), and the HIV-1 viral load was 6.4×10⁴copies/mL. Western blot (WB) analysis did not initially confirm HIV-1 infection ; however, HIV-1 infection was confirmed one month after presentation. Although several pathogens can cause infections presenting as IM, in Japan there have been few reports of acute retroviral syndrome presenting as IM. This case is being reported to stress the importance of the early diagnosis of acute retroviral syndrome.