OBJECTIVE The aim of this study was to evaluate relationships between the degree of resolution of the ST-segment elevation (ST segment resolution; STR) and the extent of microcirculatory dysfunction in infarct-related area (IRA) in patients with ST-segment elevation myocardial infarction (STEMI) using (13)N-ammonia positron emission tomography (N-PET). METHODS The subjects comprised 33 patients with STEMI who underwent successful reperfusion. Serial 12-lead electrocardiography (ECG) was performed at the baseline and at 100 min after reperfusion to calculate STR. The myocardial flow reserve (MFR) was assessed quantitatively using N-PET at 2 weeks after the onset. The summed defect score (SDS) of (99m)Tc-tetrofosmin myocardial perfusion imaging was used as an index of the severity of myocardial infarction. To assess the extent of post-infarct left ventricular remodeling, the changes in the LVEDVI (ΔEDVI) were also calculated. RESULTS A significant correlation of the STR to the MFR in IRA (r = 0.68, p < 0.0001) was observed. A significant correlation was also identified between the SDS and the baseline sum ST-segment elevation (r = 0.65, p < 0.0001), while no correlation was observed between the SDS and the STR. Furthermore, a significant inverse correlation of the STR with the ΔEDVI was also recognized (r = -0.58, p < 0.01). CONCLUSIONS These data indicate that STR after successful reperfusion in STEMI is closely related to the extent of microcirculatory disturbance; in other words, incomplete STR may be a marker of persistent microcirculatory dysfunction after reperfusion therapy.