Objective To investigate the changes in peripheral blood mononuclear cells [CD8+T lymphocytes and natural killer (NK) cells] and levels of perforin and granzyme B in these cells in children with infectious mononucleosis (IM) , as well as their clinical significance in liver injury. Methods A total of 60 children who met the diagnostic criteria for IM were enrolled, and 30 healthy children were enrolled as control group. With the help of cell surface markers and cytokine staining, flow cytometry was performed to analyze CD8+T lymphocytes, NK cells, and the expression of perforin and granzyme B. The t-test was used for comparison of continuous data between groups;a one-way analysis of variance was used for comparison between three groups, and the Dunnett-t test was used for further comparison between two groups. Results Among the 60 children with IM, the incidence rate of liver injury was 50% (30/60) ; 18 (60%) children had an alanine aminotransferase (ALT) level of < 200 IU/L, 10 (33. 3%) had an ALT level of ≥200 IU/L and < 400 IU/L, and 2 (6. 67%) had an ALT level of ≥400 IU/L. All children had normal liver functions after one month of treatment. Compared with the control group, the non-liver injury IM group and the liver injury IM group had significant increases in the expression of perforin and granzyme B in CD8+T lymphocytes (all P < 0. 05) . Compared with the non-liver injury subgroup of IM patients, the liver injury subgroup had a significantly higher percentage of CD8+T lymphocytes and significantly higher expression of perforin and granzyme B in NK cells (all P < 0. 05) . Conclusion There is a high incidence rate of liver injury in children with IM, mainly mild or moderate elevation of aminotransferases, which are self-limiting and can be returned to normal. High levels of perforin and granzyme B in NK cells and a high percentage of CD8+T lymphocytes are the cause of liver injury.