Abstract: OBJECTIVE To explore the expression of serum microRNA-21（miRNA-21） in patients with acute myocardial infarction（AMI） and pulmonary infection. METHODS A total of 80 patients with AMI and pulmonary infection who were admitted to the First Affiliated Hospital of Hainan Medical College from Feb 2018 to Oct 2020 were selected as infection group, 76 patients only with AMI were selected as non-infection group, and 57 healthy individuals underwent physical examination during the same period were selected as the control group. The expression levels of serum miRNA-21 and inflammatory factors were compared among each groups. Human cardiomyocyte（HCM） cell lines with miRNA-21 overexpression were constructed, and the apoptosis of cardiomyocytes was analyzed. RESULTS The levels of C-reactive protein（CRP）, tumor necrosis factor α（TNF-α）, interleukin-6（IL-6）, interleukin-1β（IL-1β） and miRNA-21 in the infection group were significantly higher than those in the non-infection and the control group（P＜0.05）. The levels of serum inflammatory factors including CRP, TNF-α, IL-6 and IL-1β, and the levels of miRNA-21 of patients with grade II cardiac function were significantly lower than those of patients with grade III and IV cardiac function（P＜0.05）. After the treatment with TNF-α, the apoptotic rate of cardiomyocytes in miRNA-21 overexpression group（minic group） was lower than that in the shRNA control group and HCM group（P＜0.05）. In the infection group, the levels of serum inflammatory factors of patients with mixed infection were significantly higher than those of fungal and bacterial infection（P＜0.05）, while serum miRNA-21 levels of patients with bacterial infection and fungal infection were significantly lower than that of patients with mixed infection（P＜0.05）. The levels of serum inflammatory factors and miRNA-21 in patients with low-risk to infection were significantly lower than that of patients with immediate-risk and high-risk to infection（P＜0.05）. CONCLUSION MiRNA-21 could inhibit cardiomyocyte apoptosis mediated by TNF-α. The expression level of serum miRNA-21 was elevated in patients with AMI and pulmonary infection, which was related to the cardiac function, infection type, severity of infection and inflammatory factors of patients.