Aims microRNA (miRNA) is reported to be present in the blood of humans and has been increasingly suggested as a biomarker for diseases. We aim to determine the potential of cardiac-specific miRNAs in circulation to serve as biomarkers for acute myocardial infarction (AMI). Methods and results By verifying their tissue expression patterns with real-time polymerase chain reaction (PCR) analysis, muscle-enriched miRNAs (miR-1, miR-133a, and miR-499) and cardiac-specific miR-208a were selected as candidates for this study. With miRNA microarray and real-time PCR analyses, miR-1, miR-133a, and miR-499 were present with very low abundance, and miR-208a was absent in the plasma from healthy people. In the AMI rats, the plasma levels of these miRNAs were significantly increased. Especially, miR-208a in plasma was undetected at 0 h, but was significantly increased to a detectable level as early as 1 h after coronary artery occlusion. Further evaluation of the miRNA levels in plasma from AMI patients (n = 33) demonstrated that all four miRNA levels were substantially higher than those from healthy people (n = 30, P < 0.01), patients with non-AMI coronary heart disease (n = 16, P < 0.01), or patients with other cardiovascular diseases (n = 17, P < 0.01). Notably, miR-208a remained undetectable in non-AMI patients, but was easily detected in 90.9 AMI patients and in 100 AMI patients within 4 h of the onset of symptoms. By receiver operating characteristic curve analysis, among the four miRNAs investigated, miR-208a revealed the higher sensitivity and specificity for diagnosing AMI. Conclusion Elevated cardiac-specific miR-208a in plasma may be a novel biomarker for early detection of myocardial injury in humans. © The Author 2010.