OBJECTIVES: To explore the clinical significance and correlation of microRNA-21 (miR-21) and the neutrophil-lymphocyte ratio (NLR) in patients with acute myocardial infarction (AMI). METHODS: The observation group contained 184 patients, while the control group contained 150 patients. The expression of miR-21 in the serum of each group was detected by qRT-PCR. RESULTS: A total of 184 patients and their family members were followed-up for 30 days, among which 35 patients died and 149 patients survived, resulting in a survival rate of 80.97%. According to univariate analysis, there were significant differences in age, cardiac troponin (cTn), heart rate, Killip grade, percutaneous coronary intervention (PCI) operation rate, miR-21 and NLR. In the receiver operating characteristic (ROC) analysis, the area under the curve (AUC) values of miR-21 and NLR for the diagnosis of AMI were 0.909 and 0.868, respectively, and the area under the combined detection curve was 0.960. In the Kaplan-Meier survival analysis, the survival of patients with high miR-21 expression and NLR was significantly higher than that of patients with low miR-21 expression and NLR (p=0.027; p=0.001). The correlation showed that miR-21 expression in serum was positively correlated with the NLR in the observation group (r=0.528, p<0.05). cTn, heart rate, Killip classification, PCI operation rate, miR-21, NLR are independent risk factors for AMI. CONCLUSION: miR-21 and NLR play a role in the diagnosis of AMI and can be used as predictors for the survival of AMI.