A 26-year-old female was admitted because of multiple fractures in lower extremities. While in the hospital, she developed a high fever and generalized skin eruption. Physical examination revealed bilateral cervical lymphadenopathy and mild hepatosplenomegaly. The white cell count was 11,200 with 11% atypical lymphocytes. Serum GOT, GPT, LDH were markedly elevated. Infectious mononucleosis was suspected, but the serological test for EB virus did not show evidence of acute EB virus infection. Anti-HSV, CMV, hepatitis A virus antibody titers also did not show significant change during the coarse. The serological test for HHV-6 only showed increased titer of IgM and IgG antibodies. Rapidly elevated IgG antibody titer was indicative of reactivation of HHV-6. So, she was diagnosed as mononucleosis-like syndrome caused by HHV-6, probably reactivated infection. Her symptoms gradually disappeared during a month.