MicroRNAs (miRNAs) are a novel class of small, non-coding RNAs that play a significant role in both inflammatory and cardiovascular diseases. Immune cells, especially T helper (Th) cells, are critical in the development of atherosclerosis and the onset of acute coronary syndrome (ACS). To assess whether inflammation-related miRNAs (such as miR-155, 146a, 21, 125a-5p, 125b, 31) are involved in the imbalance of Th cell subsets in patients with ACS, we measured the expression of related miRNAs in patients with acute myocardial infarction (AMI), unstable angina (UA), stable angina (SA) and chest pain syndrome (CPS); analyzed the relationship between miRNA expression and the frequency of Th cell subsets; and observed the co-expression of miR-155 and IL-17A in peripheral blood mononuclear cells (PBMCs) of patients with ACS. The results showed that the expression of miR-155 in the PBMCs of patients with ACS was decreased by approximately 60%, while the expression of both miR-21 and miR-146a was increased by approximately twofold. The expression patterns of miRNAs in plasma correlated with those in PBMCs, except for miR-21, which was increased by approximately sixfold in the AMI group and showed no significant difference between the UA group and the CPS group. We also found that the expression of miR-155 inversely correlated with the frequency of Th17 cells (r=-0.896, P<0.01) and that miR-155 was co-expressed with IL-17A in patients with ACS. In conclusion, our study revealed the expression patterns of inflammation-related miRNAs in patients with ACS and found that miR-155 may be associated with Th17 cell differentiation.