Abstract: OBJECTIVE To explore the change of adenosine deaminase (ADA) among the different age groups of children with Mycoplasma pneumoniae infection and analyze its association with immune function. METHODS A total of 150 children with M. pneumoniae infection who were treated in the hospital from Jul. 2023 to Mar. 2024 were recruited as the research subjects and were divided into the 1-4 years old group with 39 cases, the 4-8 years old group with 52 cases and the 8-12 years old group with 59 cases according to the age. The clinical data and clinical laboratory test indexes were observed and compared among the groups. The association between ADA and immune function indexes was analyzed by Pearson correlation. RESULTS There were significant differences in the pulmonary imaging change, length of hospital stay, total fever duration, time of disappearance of cough, time of disappearance of pulmonary rales, CD₃⁺, CD₄⁺, CD₈⁺, CD₄⁺/CD₈⁺, while blood cell (WBC) counts, D-dimer(DD), lactic dehydrogenase(LDH), ADA, alanine aminotransferase(ALT), aspartate transaminase(AST), neutrophils counts, lymphocytes counts and platelet level among the three age groups of children after the M. pneumoniae infection(P＜0.05). The ADA level was (28.12±5.98)U/L in the 1-4 years old group, (30.32±5.91)U/L in the 4-8 years old group,(33.30±5.96)U/L in the 8-12 years old group. The serum ADA level of the different age groups of children was positively correlated with CD₄⁺(r=0.289 to 0.660) and CD₄⁺/CD₈⁺(r=0.389 to 0.530) but was negatively correlated with CD₈⁺(r=-0.431 to -0.417)(all P＜0.05). CONCLUSIONS The ADA level can be used for the assessment of treatment outcomes of the different age groups of children with M. pneumoniae infection, and it is closely associated with the immune function.