Background: In patients with acute myocardial infarction (AMI), it is important to predict the prognosis as early as possible. The aim of this study is to evaluate the usefulness of the index of microcirculatory resistance (IMR) obtained immediately after reperfusion for predicting myocardial viability and left ventricular (LV) function recovery in patients undergoing primary percutaneous coronary intervention (PCI) for AMI. Methods: After successful primary PCI in 36 patients with anterior AMI, IMR was measured using a pressure-temperature sensor-tipped coronary guidewire. Myocardial viability was quantified by the relative uptake of 18F-fluorodeoxyglucose (FDG) imaged with positron emission tomography (PET) at 8짹1 days after primary PCI. Echocardiographic regional wall motion was analyzed to calculate the anterior wall motion score (A-WMS) and percent change in A-WMS after revascularization and at 6-month follow-up. Results: The IMR correlated significantly with the regional myocardial FDG uptake (r = -0.695, p < 0.001) and demonstrated significant correlation with percent change in A-WMS (r = 0.405, p = 0.012). The area under the receiver operating characteristic curve of IMR for predicting LV wall motion recovery was 0.86. Multivariate analysis revealed IMR was the only significant predictor of LV wall motion recovery at 6-month follow-up [odds ratio (OR) = 0.915, 95% CI = 0.890–0.975, p = 0.015]. Conclusions: IMR, a new index representing microvascular integrity, is a reliable early on-site determinant of myocardial viability and LV recovery just after primary stenting for AMI.