Introduction:
The objective of the current study was to assess the serum level of interleukin‑18 (IL-18) in children with mononucleosis and to compare the expression of this cytokine in the acute and past phase of the infection.

Material and methods:
The level of IL-18 was determined using commercial enzyme-linked immunosorbent assay kits (Cloud-Clone Corp China). Other laboratory findings (white blood cells – WBC, C-reactive protein – CRP, alanine aminotransferase – ALT, aspartate transaminase – AST, lactate dehydrogenase – LDH, γ-glutamyl transferase – GGT) were determined using routine laboratory methods using a Cobas c501 and Sysmex 4000i.

Results:
The level of IL-18 was significantly higher in the acute phase compared to the past stage of the disease (45.3 pg/ml vs. 21.6 pg/ml; p = 0.001) and in the acute phase compared to the control group (45.3 pg/ml vs. 23.8 pg/ml; p = 0.045).

Conclusions:
In conclusion, this study shows that serum concentration of IL-18 in children with mononucleosis in the acute phase is increased, thus confirming the ongoing inflammatory process. We also suggest that IL-18 can be a useful new marker to differentiate between the acute and past phase of the disease. The determination of IL-18 with the activities of AST, ALT, LDH, GGT, leukocytosis (WBC) and the level of acute phase protein (CRP) may help assess the phase of the disease.