Purpose: MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression by interacting with multiple mRNAs. miRNAs are also released from tissue into plasma as a consequence of damage and appear to reflect both the degree of damage seen in tissues, including the heart, and also long term remodelling. The aim of this work was to measure miRNAs in the plasma of ST elevation myocardial infarction (STEMI) patients who were undergoing primary percutaneous coronary intervention (PPCI).Methods: 50 patients admitted to hospital with STEMI and who were undergoing PPCI were recruited. Plasma extracted from blood samples that were collected from the patients prior to PPCI (baseline), 24 hours after PPCI and 3 months after PPCI was stored at -80°C. Total RNA was extracted from the plasma using miRNeasy serum/plasma kit (Qiagen). miRNAs were measured using TaqMan primers and miRNA levels were assessed using quantitative PCR. Quantification was performed using cel-miR-39-3p (000200) as the normalising miRNA. miRNAs measured were hsa-miR-133a-3p (002246), hsa-miR-194-5p (000493), hsa-miR-208b-3p (002290) and hsa-miR-214-3p (002306) (Life Technologies).Results: Both miRNA-133a and miRNA-208b which have been shown to increase following reperfusion injury were significantly elevated at 24 hours compared to both baseline and 3 month time points. After 3 months miRNA-208b was undetectable in most patients. In contrast, miRNAs associated with heart failure (miRNA-194 and miRNA-214) did not show significant changes throughout. However, when looking at individual patients 60 % of them had elevated levels of miRNA-194 and miRNA-214 after PPCI.Conclusions: These data suggest that after reperfusion there is a release of both miRNA-133a and miRNA-208b into the bloodstream. Even before reperfusion there is an increase in these miRNAs suggesting that there is cellular damage prior to PPCI. Both miRNA-194 and miRNA-214 have implications in cardiac dysfunction and protection but it appears these are not released into the plasma as a consequence of cardiac damage. The different patient profiles of miRNA-194 and miRNA-214 with respect to long term patient outcome are currently being investigated.