We compared the accuracy in predicting regional wall motion score index (RWMSI) changes between microvascular integrity indexes measured during primary percutaneous coronary intervention (PCI) and fluorine-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) in ST-elevation myocardial infarction (STEMI). Fifty patients with STEMI were enrolled. Microvascular integrity indexes were measured using an intracoronary Doppler wire and a pressure wire after primary PCI. We performed FDG-PET 7 days after PCI. RWMSI on follow-up echocardiogram (5.8 +/- 1.7 months) revealed good correlations with coronary flow reserve (r = -0.442, p = 0.002), diastolic deceleration time (r = -0.511, p <0.001), microvascular resistance index (r = 0.443, p = 0.002), coronary wedge pressure (r = 0.474, p <0.001), and FDG uptake rate (r = -0.571, p <0.001). There were no significant differences in areas under the curve for predicting RWMSI changes between microvascular integrity indexes and FDG-PET (coronary flow reserve 0.696, diastolic deceleration time 0.731, microvascular resistance index 0.748, coronary wedge pressure 0.694, Thrombolysis In Myocardial Infarction myocardial perfusion grade 0.702, and FDG-PET 0.755). In conclusion, microvascular integrity indexes assessed during primary PCI are useful and comparable to FDG-PET in predicting left ventricular functional changes in STEMI.