Illnesses resembling mononucleosis, hematologically characterized by atypical lymphocytosis in the peripheral blood, are caused by other viral infections as well as by a primary Epstein-Barr virus infection. Human herpesvirus 6, a newly isolated member of the herpesvirus group, can also cause a mononucleosis-like illness. Illness associated with human herpesvirus 6 infection mostly occurs in immunocompetent adults. We observed a 3 month old infant who presented with marked atypical lymphocytosis and liver dysfunction. We examined serum samples to detect viral antibodies related to mononucleosis-like illness. Only the titers of antibody against human herpesvirus 6 were elevated. Primary human herpesvirus 6 infection cannot only cause exanthem subitum or present in an inapparent form but can also cause an illness like mononucleosis, even in an infant.