Objective:To investigate the level of cardio-specific microRNA-499 levels (miR-499) in acute myocardial infarction (AMI) patients and to explore the effect of plasma miR-499 levels on the diagnosis of AMI. Methods: The subjects enrolled in this study, included 30 healthy subjects and 73 patients with suspected acute coronary syndrome (ACS),who went on our hospital in 2013 and were divided into the AMI group and the unstable angina (UA) group with 53 and 20 patients, respectively. MiR-499 concentrations were measured with a real-time reverse-transcription PCR (qRT-PCR). Blood samples of the AMI group were collected 12 h and 24 h after the onset of symptom, and those of the UA group were collected after admission. Serum cardiac troponin I (cTnI ) and creatine kinase-MB (CK-MB) concentrations were measured by the electrochemiluminescence method. The 53 AMI patients were further divided into various subgroups according to coronary arteries involved and primary PCI or not, plasma miR-499 concentrations were measured by TaqMan real-time PCR and analyzed between the two groups. Results: The relative level of miR-499 was significantly increased in patients with AMI (4.57 ± 2.3) than that of UA subjects (2.75 ± 1.39) and normal subjects (0.5 ± 0.39) (both P < 0.01). The serum miR-499 relative level in the patients with AMI had a positive correlation with serum cTnI or CK-MB (r = 0.361, 0.428, respectively, both P < 0.01). It was significantly higher in 32 AMI patients with double or triple coronary arteries stenosis than those with single stenosis (P < 0.01). MiR-499 was significantly decreased in 24 patients after received primary PCI (P < 0.01). Conclusion:The plasma concentration of miRNA-499 with cardiac specific expression is low in healthy person, but significantly increase after myocardial infarction and can be detected in the early stage. This can be used for early diagnosis and AMI severity evaulation. The plasma concentration of miR-499 may be a useful biomarker of AMI in humans.