Background Hyperglycemia has been shown to be a powerful predictor of poor outcome after ST-segment elevation myocardial infarotion (STEMI). This study aimed to evaluate the effect of admission glucose on microvascular flow after successful primary percutaneous coronary intervention (PCI) in patients with STEMI.Methods Successful primary PCI was performed in 267 patients with STEMI. The maximum ST elevation of single electrocardiogram (ECG) lead before and 60 minutes after PCI was measured, and patients were then divided into 3 groups according to the degree of ST-segment resolution (STR): absent (＜30%), partial (30% to 70%) or complete(≥7o%).Results Of the 267 patients, 48 (18.0%) had absent STR, 137 (51.3%) experienced partial STR, and 82 (30.7%) had complete STR. The degree of STR decreased with increasing admission glucose levels (P=0.032), and patients with hyperglycemia (serum glucose level ≥11 mmol/L) were more likely to have absent STR (P=0.001). Moreover,hyperglycemia was an independent predictor of incomplete STR (odds ratio, 1.870; 95% confidence interval, 1.038 to 3.371, P=0.037).Conclusions Hyperglycemia on admission is associated with abnormal coronary microvascular reperfusion in patients with STEMI after successful primary PCI, which may contribute, at least in part, to the poor outcomes in these patients.