Рurpose: to identify clinical and laboratory criteria that mark the severity of infectious mononucleosis in children, with the identification of risk factors for the development of severe forms and unfavorable course.Patients and methods. A comparative prospective clinical study involved 200 children aged 3 to 11 years with moderate (125 people) and severe (75 people) form of infectious mononucleosis. When comparing clinical and laboratory data in the compared groups, the Mann-Whitney test was used. For multiple comparisons, the Kruskal-Wally test was used. Significance of differences was determined at a significance level of 0.05 (p < 0.05).Results. A comparative analysis of the clinical and laboratory manifestations of infectious mononucleosis revealed statistically significant differences in the studied parameters depending on the severity of the disease. Severe forms of infectious mononucleosis were distinguished by the most pronounced clinical symptoms, as well as changes in hematological parameters and biochemical changes. The combined etiology of infectious mononucleosis also contributed to a more severe course of the disease.Сonclusion. Timely diagnosis and assessment of the severity of the manifest form of infectious mononucleosis determine the adequacy of the appointment of rational antiviral and pathogenetic therapy, preventing the development of adverse effects and complications.