Purpose: MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression by interacting with multiple mRNAs. They can be released from tissue into plasma as a consequence of damage and appear to reflect the degree of damage seen in tissues including the heart. The released miRNAs are often monitored in plasma and not in whole blood where they can be incorporated into different blood constituents. The aim of this work was to determine whether miRNAs measured in whole blood reflect plasma levels. The miRNAs investigated were miRNA-133a and miRNA-208b which have been shown to be released from damaged cardiac tissue, miRNA-194 is linked with the development of heart failure and miRNA-214 is associated with the failing heart, cardiac hypertrophy and is released from rat hearts into the plasma in response to acute myocardial infarction.Methods: 18 patients admitted to hospital with acute ST elevation myocardial infarction (STEMI) who were undergoing primary percutaneous coronary intervention (PPCI) were recruited. Blood samples were collected for either plasma or whole blood miRNA extraction from patients 24 hours after PPCI. Total RNA was extracted from the plasma using miRNeasy serum/plasma kit (Qiagen) and RNA from whole blood was collected and extracted using PAXgene blood miRNA 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). The peak troponin I measurement was taken at 12 hours post PPCI.Results: Both miRNA-133a and miRNA-208b levels in plasma, but not whole blood, showed a significant positive correlation with peak troponin I release. In contrast, peak troponin I release showed a positive correlation with miRNA-194 and miRNA-214 levels in whole blood but not in plasma. Conclusions: These data shows that reperfusion injury-related miRNAs (133a and 208b) in plasma correlated with cardiac injury as measured by troponin I levels. However, troponin I levels correlated with miRNAs related to cardiac remodelling (e.g. failure) measured in whole blood but not in plasma. Whether the miRNA levels in plasma or in whole blood are indicative of long term outcome is currently being investigated.