Background : The state of coronary microcirculaton is an important determinant of myocardial viability and clinical outcomes in acute myocardial infarction (AMI). However, to date, a simple and invasive method for assessing the integrity of coronary microcirculation and myocardial viability in AMI is lacking. Recently, a novel index of microcirculatory resistance (IMR) has been shown to correlate well with true microvascular resistance. We assessed the usefulness of IMR as a predictor for myocardial viability after primary percutaneous coronary intervention (PCI) in AMI. Methods : We studied 17 patients (15 male, mean age 55짹12 years) underwent primary PCI for AMI (LAD:11, RCA:5, LCX:1) within 24 hours from the onset of symptoms. After successful PCI, by using a pressure-temperature sensor-tipped coronary wire, thermodilution-derived coronary flow velocity reserve (CFR_thermo), fractional flow reserve (FFR) and coronary wedge pressure (P_cw) were measured and the ratio of P_cw and mean aortic pressure (P_cw/P_a) was calculated, along with IMR, defined as the distal coronary pressure divided by the inverse of the hyperemic mean transit time. ¹⁸F-fluorodeoxyglucose (FDG) PET was performed after primary PCI in 7 days to evaluate myocardial viability by regional percentage uptake of FDG in infarcted segments. Results : Significant linear correlation between IMR and regional uptake of FDG in infarcted segments (r=-0.548, p=0.023) was observed, and also the correlation between regional FDG uptake and P_cw/P_a was significant (r=-0.508, p=0.037), but weaker than IMR. However, regional FDG uptake did not correlate with CFR_thermo (r=0.405, p=0.106), FFR (r=-0.213, p=0.413) and P_cw (r=-0.336, p=0.187). Cut-off point of IMR for the prediction of myocardial viability as defined by the 50% FDG PET threshold value was 21.7 (78% sensitivity , 88% specificity ). Conclusions : IMR, a novel index representing the integrity of coronary microcirculation, is a relatively simple and useful parameter for the invasive assessment of myocardial viability in patients with AMI undergoing primary PCI.