Objective To study the clinical effect of pidotimod oral liquid as adjuvant therapy for infectious mononucleosis and its effect on T lymphocyte subsets.Methods A total of 76 children with infectious mononucleosis, who were admitted to the hospital between July 2016 and June 2017, were enrolled and randomly divided into two groups:conventional treatment and pidotimod treatment (n=38 each). The children in the conventional treatment group were given antiviral therapy with ganciclovir for injection and symptomatic treatment. Those in the pidotimod treatment group were given pidotimod oral liquid in addition to the treatment in the conventional treatment group. The course of treatment was two weeks for both groups. The two groups were compared in terms of the recovery of clinical indices and the changes in peripheral blood T lymphocyte subsets.Results Compared with the conventional treatment group, the pidotimod treatment group had signifcantly shorter fever clearance time, time to the disappearance of isthmopyra, time to the relief of lymph node enlargement, time to the relief of hepatosplenomegaly, and length of hospital stay (P < 0.05). After treatment, the pidotimod treatment group had signifcant reductions in the percentages of CD3⁺ and CD8⁺ T cells and had signifcantly lower percentages of CD3⁺ and CD8⁺ T cells than the conventional treatment group (P < 0.001). The pidotimod treatment group had signifcant increases in the percentage of CD4⁺ T cells and CD4⁺/CD8⁺ ratio after treatment, which was signifcantly higher than those in the conventional treatment group (P < 0.001). The conventional treatment group had no signifcant changes in T lymphocyte subsets after treatment (P > 0.05).Conclusions Pidotimod oral liquid has a good clinical effect as the adjuvant therapy for infectious mononucleosis and can improve cellular immune function, so it holds promise for clinical application.