Epstein-Barr virus (EBV) infections in children are usually asymptomatic and occur in almost the entire population. If the infection occurred at a later age, in adolescents and adults, it can lead to infectious mononucleosis (IM). A typical symptom is long-lasting fever, which is of concern, especially in younger patients and those who do not respond to antibiotic therapy. The paper presents the characteristics, epidemiology, and pathophysiology of EBV infection. It is described both common and rare features of the most common form of symptomatic infections – infectious mononucleosis. IM is suspected primarily in children with fever, sore throat and enlarged lymph nodes, liver and spleen. EBV infection can cause mucocutaneous manifestation, weakness and myalgia; hepatitis is relatively less common and can lead to liver failure. Laboratory diagnosis is primarily an assessment of morphology and peripheral blood smear, with the presence of the predominant atypical lymphocytes. Evaluation of serum transaminases level (elevated in infectious mononucleosis), and serological tests for the presence of antiviral antibodies are also crucial. If the heterophil test is negative in the patients with symptoms suggesting IM, one should carry out EBV-specific antibody tests. Antibodies to viral capsid antigen (VCA) appear as the first. The paper reports three cases of infectious mononucleosis and diagnostic problems associated with them.