Background: Recent studies show that microRNA-145 (miRNA-145) might be an attractive tumor biomarker of considerable prognostic value, but little is known about their relationship with acute myocardial infarction (AMI). This study investigated the correlation between the level of miR-145 and AMI. Methods: One-hundred patients were divided into three groups: no coronary artery disease (CAD) group, non-ST segment elevation myocardial infarction group, and ST segment elevation myocardial infarction group. The plasma levels of miR-145 were quantified using real-time quantitative polymerase chain reaction. Logarithmic transformation of miRNA-145 levels (Ln_miRNA-145) was used for statistical analysis due to the skewed data distribution. Results: Plasma levels of miR-145 were significantly lower in patients with AMI compared to patients in the non-CAD group (-6.38 +/- 0.11 vs. -4.47 +/- 0.17, P < 0.0001). Compared to those without heart failure, the levels of miR-145 were significantly lower in patients with heart failure (6.91 +/- 0.20 vs. 5.35 +/- 0.13, P < 0.0001). We also found that the lower plasma levels of miRNA-145 significantly correlated with increased serum levels of B-type natriuretic peptide (Spearman rho = -0.60, P < 0.0001), troponin T (Spearman rho = 0.62, P < 0.0001), and decreased ejection fraction (Spearman rho = 0.65, P < 0.0001). In a multivariable linear regression analysis, AMI and heart failure were independently associated with lower Ln_miRNA-145 (estimate -0.99, standard error [SE] 0.28; P = 0.001 and estimate 0.62, SE 0.21; P = 0.004). Conclusions: Our results suggest that decreased plasma levels of miR-145 are associated with AMI. Circulating miR-145 may be useful in prognosticating cardiac function and the risk of developing heart failure.