We analyzed anti-Cytomegalovirus (CMV) IgG and IgM antibody (EIA) and anti-Epstein-Barr virus (EBV) viral capsid antigen (VCA) IgG and IgM antibody (FA) in adults during 1994-1999. We examined these IgM sero-positive patient's medical records, and diagnosed CMV mononucleosis and EBV mononucleosis. Anti-CMV antibody positive rates decreased from 87.6% in 1994 to 77.8% in 1999. Especially in twenties, anti-CMV antibody positive rates decreased from 65.2% in 1994 to 53.3% in 1999. On the other hand, anti-EBV VCA antibody positive rates were not changed (91-94%). Number of cases of CMV mononucleosis increased from 2 cases in 1994 to 16 cases in 1999, but EBV mononucleosis was not changed. These results suggested that increasing cases of CMV mononucleosis was influenced by decreasing anti-CMV antibody positive rate.