Introduction. A meta-analysis has been performed to assess the results of the peptide medication Thymalin application in patients suffering from different pathologies accompanied by immunodeficiency (decreased count of lymphocytes, T-lymphocytes). Thymalin restores immunity, regulates the number and the ratio of T- and B-lymphocytes and their subpopulations, stimulates cell immunity response, increases phagocytosis, facilitates the processes of regeneration and haemopoiesis in case of their suppression and enhances the processes of cell metabolism. Therefore, the medication is used in cases of various of immunodeficiences and illnesses accompanied by immune disorders. The activity of Thymalin was found to be based on the effects of the constituent peptide components which have certain mechanisms of action and entry points. Ultra-short peptides composing Thymalin are able to produce a specific impact on different elements of immunity thus providing immunomodulating properties of Thymalin. The work is aimed to identify a statistically significant change in the count of T-lymphocytes in patients treated with Thymalin as compared to those in the control group. Materials and Methods. A meta-analysis largely combines the results of randomized controlled studies in which clinical effects obtained in the main groups and cotrolled groups are being compared. Four publications focused on the use of Thymalin in order to correct immunodeficiencies accompanying different pathologies have been selected for the purpose. In all papers the level of T-lymphocytes has been assessed as an index of immunodeficiency pronouncement. The proportions of the change of this index in the main group (with Thymalin) and a control group (without Thymalin) were compared before and after treatment. R-language software environment being an instrument for statistical computing of medical data has been used in the study. Results and Discussion. The results of meta-analysis are given in the forestplot named blobbogram. Meta-analysis machine chart has been also provided. Verification of statistical hypothesis regarding clinical effects is made on the basis of the p rate significance, however besides a conclusion on the presence or absence of the effect, based on the Standard Mean Differences (SMD) one can estimate its value as well. Classification of the effect size may be expressed in the SMD values: «small» - 0,2; «medium» - 0.5, «large» - 0,8; «very large» - 1.3. This classification allows to get an overall picture about the effect size to be accounted in the context of a clinical target. Conclusion. Patients administered with Thymalin showed a statistically significant increase in the initially reduced count of T-lymphocytes as compared to those in control group. Thymalin mediated effect size has been classified as a “large” one (SMD>0.8) in compliance with accepted methodologies in the field of statistics.