Background Concerns exist about reliability of pressure-wire-guided coronary revascularization of non-infarct-related arteries (non- IRA ). We investigated whether physiological assessment of non- IRA during the subacute phase of myocardial infarction might be flawed by microcirculatory dysfunction. Methods and Results We analyzed non- IRA that underwent fractional flow reserve, coronary flow reserve, and the index of microcirculatory resistance assessment. Microcirculation and hyperemic response were evaluated in 49 acute myocardial infarction patients (59 non- IRA ) and compared with a matched control group of 46 stable angina ( SA ) patients (59 vessels). Time between acute myocardial infarction to physiological interrogation was 5.9±2.4 days. Fractional flow reserve was similar in both groups (0.79±0.11 in non- IRA versus 0.80±0.13 in SA vessels, P=0.527). Lower coronary flow reserve values were found in non- IRA compared with SA vessels (1.77 [1.25-2.76] versus 2.44 [1.63-4.00], P=0.018), primarily driven by an increased baseline flow in non- IRA (rest mean transit time 0.58 [0.32-0.83] versus 0.65 s [0.39-1.20], P=0.045), whereas the hyperemic flow was similar (hyperemic mean transit time 0.26 [0.20-0.42] versus 0.26 s [0.18-0.35], P=0.873). No differences were found regarding index of microcirculatory resistance (15.6 [10.4-21.8] in non- IRA versus 16.7 [11.6-23.6] U in SA vessels, P=0.559). During adenosine infusion, the hyperemic response was similar in both groups (non- IRA versus SA vessels) in terms of the resistive reserve ratio (3.1±2.1 versus 3.7±2.2, P=0.118). Conclusions In the subacute phase of myocardial infarction, non- IRA show an increased baseline flow that may cause abnormal coronary flow reserve despite preserved hyperemic flow. In non- IRA , microcirculatory resistance and adenosine-induced hyperemic response are similar to those found in SA patients. From a physiological perspective, these findings support the use of fractional flow reserve to interrogate non- IRA during the subacute phase of myocardial infarction.