【Abstract】 Objective To analyze the value of plasma microRNA21 (miR21) in the early daignosis and shortterm prognosis of aucte myocardial infarction. Methods Sixtytwo patients with acute myocardial infarction (AMI group, n=62) were enrolled in the study. Plasma miR21 levels were measured by quantitative reverse transcription polymerase chain reaction (qRTPCR) within 6h,12h, 24h, 7d and 14d after the onset of acute chest pain. Plasma cTnT levels were measured by electrochemiluminescencebased methods. The major adverse cardiovascular events (MACE) were recorded during the followup of 6 months. In the same period, 30 healthy subjects (control group, n=30) were enrolled. Results The level of cTnT within 6 hours of symptoms onset in AMI group was significantly higher than that in control group (P＜001). The level of miR21 within 6 hours of symptoms onset in AMI group was significantly higher than that in control group (P＜001). miR21 level began to increase within 6 hours of symptoms onset, reached a peak at 12h,and decreased gradually to normal at 24h, 7d and 14d. cTnT level began to increase within 6h after onset,reached a peak at 24h,and decreased gradually to normal at 7d and 14d. The area under curve (AUC) for miR21 within 6 hours of symptoms onset was 0939 (95%CI:0893～0984,P=0000).The AUC for miR21 at 12h was 0956 (95%CI:0918～0994,P=0000);The AUC for cTnT within 6 hours of symptoms onset was 0964 (95%CI:0932～0996,P=0000).The AUC for cTnT at 12h was 0979 (95%CI:0956～1000,P=0000). After followingup for 6 months, there was a statistically significant difference in the level of plasma miR21 expression between patients with MACE and patients without MACE at 6h,12h,24h,7d and 14d (all P＜001). Conclusion The level of miR21 has a certain reference value for the early diagnosis and prognosis of AMI, but it is not superior to cTnT.