Objectives: Fractional flow reserve(FFR) and hyperemic epicardial stenosis resistance index(hESRI) have been applied to evaluate the hemodynamic severity of epicardial coronary stenosis in patients with angina. However, the usefulness of FFR and hESRI in patients with AMI are not clearly defined. Methods: Sixty-five intermediate lesions of 57 patients(ages: 59짹11, male: 50, IRA:17) were studied. IVUS % area stenosis(r-%AS), FFR, coronary flow reserve(CFR) and hESRI were measured. All lesions were divided into two groups(Group 1(n=48); non-IRA, Group 2(n=17); IRA) Results: There was not a significant difference in r-%AS(%) between two groups(group 1 vs. group 2: 75.8짹7.7 vs. 74.9짹10.1, p=0.752). CFR was significantly higher in group 1(3.49짹1.00 vs. 2.18짹0.57, p<0.001). FFR was significantly higher in group 2(0.79짹0.12 vs. 0.71짹0.14, p=0.041). hESRI was lower in Group 2 than Group 1, although the value was not significant statistically(0.63짹0.45 vs. 0.97짹0.79, p=0.117). In group 1, Best cut-off values(BCV) of FFR and hESRI for 75% of r-%AS were 0.75 (sensitivity 86.2%, specificity 84.2%, AUC 0.884) and 0.64(sensitivity 79.3%, specificity 84.2%, AUC 0.860), respectively. However, in group 2, BCV of FFR and hESRI for 75% of r-%AS were 0.83(sensitivity 70.0%, specificity 71.4%, AUC 0.750) and 0.41(sensitivity 87.5%, specificity 71.4%, AUC 0.786), respectively. Concordance rate between FFR and r-%AS in group 1 was 83.3%(sensitivity 85.7%, specificity 80.0%, p<0.001; 觀=0.657, p<0.001). That was 64.7%(sensitivity 40.0%, specificity 100%, p=0.088; 觀=0.354, p=0.056) in group 2. Conclusion: FFR was underestimated for the assessment to the functional severity of coronary artery stenosis in patients with AMI. New cut-off value of FFR might be required for the evaluation of the significant coronary artery stenosis in AMI.