Summary. The spleen is a secondary immune and the largest organ of the reticulo-endothelial system, actively involved in the process of infectious mononucleosis. At the clinical level, an assessment of the degree of organ involvement in the pathological process is unlikely. As a rule, for the diagnosis of splenomegaly, only palpation and percussion determination of the size of the spleen is used, which is a late and subjective sign of possible splenomegaly. Ultrasound examination provides ample opportunity to study the size of the spleen in infectious mononucleosis. The purpose of this study was to identify morphometric and dopplerographic changes in the spleen in patients with infectious mononucleosis. Materials and methods. The study group consisted of 24 patients with infectious mononucleosis. The control group is represented by 30 healthy students of the medical institute. All participants in the study underwent ultrasound examination of the spleen. Echostructure of the organ, clarity of the contour was evaluated, linear dimensions were measured: length, width, thickness of the spleen. The speed indicators in the splenic artery and vein in the gates of the spleen were measured. The positioning of the elastographic window during fibroelastometry was performed in the standard seroscale examination window. Measurements were taken at 5 or more points of the spleen parenchyma with a distance of at least 4-5 mm from the capsular zone and the zone of large vessels. Based on the obtained morphometric measurements, the mass and spleen mass coefficient (CMS), the ratio of spleen mass and growth and body surface area were calculated. Results. The study found that in patients with infectious mononucleosis, the length, thickness and weight of the spleen are statistically significantly greater than in healthy ones. But the mass of the organ varies widely. Therefore, CMS was calculated, which is an objective criterion for the size of the spleen. It was revealed that with infectious mononucleosis there are three variants of the spleen response: decrease, CMR less than 1.5; normal value, CCM is in the range from 1.5 to 4; splenomegaly, CMS more than 4. Splenomegaly is associated with increased organ stiffness, peak blood flow velocity and pressure in v. Lienalis, as well as periportal lymphadenopathy. Identified changes are characteristic of young patients. Conclusion. Ultrasound examination of the spleen is of high diagnostic value in patients with infectious mononucleosis. Using the method, an accurate assessment of the state of the immune organs in the acute period of the disease is possible. At the preclinical level, gradation of splenomegaly is possible depending on its severity, assessment of hemodynamics and organ stiffness.