The aim of the study was to assess the dynamics of changes in magnesium (Mg) serum levels in the acute, symptomatic period of infectious mononucleosis and during the convalescence period, when clinical symptoms were no longer present. 50 subjects were included, among them 26 patients (14 women and 12 men) at the age of 16 to 27 years in the acute, symptomatic stage of the disease. The diagnosis of infectious mononucleosis was based on the clinical, hematological, biochemical and serologic criteria. Mg concentration was measured 3 times: on the 1st day and in the 2nd week of hospitalization, and also in the convalescence period--3 weeks after the regression of clinical symptoms. The control group consisted of 24 healthy individuals (5 men and 5 women) at the age of 17 to 26 years. Mg concentration was measured once in this group. All measurements of Mg serum concentration were performed using the method of atomic absorption spectrophotometry (AAS) at the wavelength of 285.2 nm. We observed significantly higher Mg serum concentrations in patients with acute, symptomatic infectious mononucleosis than in healthy individuals. During the convalescence period Mg serum concentrations were lower but still statistically higher than in the control group.