Primary Epstein-Barr virus (EBV) and cytomegalovirus (CMV) infection usually affects preadolescent children or young adults, causing similar clinical presentation. Signs and symptoms are typically mild, and the majority of clinical and laboratory findings resolve spontaneously within one month after onset. In adulthood, the risk of fulminant EBV infection and severe complications is much higher, which may be related to increasing memory CD8+ T-cell population with age. It is still not clear what exactly triggers T-cell clonal proliferation. Animals model studies on heterologous immunity between viruses revealed that pre-existing memory T-cells may contribute to excessive immune response during subsequent infection with a new, unrelated pathogen. A 3.5-year-old girl was admitted to hospital with a suspicion of lymphoproliferative disorder. Peripheral blood smear revealed a massive lymphocytosis (61,600 cells/µl) with 62% share of atypical lymphocytes. The clinical presentation and positive EBV and CMV IgM test strongly suggested infectious mononucleosis syndrome as a result of EBV and CMV coinfection.