Infectious mononucleosis caused by the Epstein-Barr virus (EBV) is a self-limiting infection in most immunocompetent hosts that is commonly transmitted by droplets of saliva from infected person. During the acute phase, EBV infects oropharyngeal epithelial cells and B-lymphocytes which are destroyed by cytotoxic T-lymphocytes. Acute phase is followed by a lifelong latent infection. Different cell subsets express on its surface a large number of specific markers which enables their identification. Flow cytometry analyzes the number and ratio of specific cell subpopulations and recognizes the pathological changes of cellular immunity which presents immunophenotypic profile. This study included subjects with a clinical diagnosis of infectious mononucleosis caused by EBV. The aim of this study was to determine immunophenotypic profile of patients with a clinical diagnosis of infectious mononucleosis using flow cytometry. Percentages of certain lymphocyte subpopulations of peripheral blood; T lymphocytes, B lymphocytes, NK cells, CD4+ T lymphocytes, CD8+ T lymphocytes, activated CD8+CD38+ i HLA-DR T lymphocytes were determined for all patients in this study and then compared with reference values for healthy individuals. Patients with EBV-induced infectious mononucleosis showed significantly increased percentages of total T lymphocytes, cytotoxic CD8+ T lymphocytes and activated CD8+CD38+ and HLA-DR+ T lymphocytes compared to reference values. Percentages of CD4+ T lymphocytes, as well as B lymphocytes, were significantly decreased. Absolute numbers of CD4+ T lymphocytes and percentages of NK cells did not show a statistically significant difference from the reference values.