Background It has been demonstrated that an elevated blood glucose level is associated with increased mortality in patients with an acute myocardial infarction (AMI). We hypothesized that admission hyperglycemia would be associated with the increased infarct size which was measured by contrast-enhanced magnetic resonance imaging (CE-MRI). Methods In 207 patients (176 males; mean age 58.6 짹 11.9 years) with an acute myocardial infarction successfully treated by early invasive intervention, CE-MRI was performed a median of 7 days after the index event. Admission hyperglycemia was defined as randomly checked blood glucose above 200 mg/dl. Infarct size was measured on delayed-enhancement imaging and area at risk (AAR) was quantified on T2-weighted images. Myocardial salvage index was computed as follows: Myocardial salvage index = (AAR – infarct size) �� 100/ AAR. Results Of 156 STEMI and 51 NSTEMI patients, 39 (18.8%) had hyperglycemia on admission. Baseline characteristics were not significantly different between the patients with glucose < 200mg/dl and ��� 200mg/dl. Compared to normoglycemic patients, patients with hyperglycemia at admission had significantly lower LV ejection fraction (52.1 짹 13.8% versus 46.7 짹 14.4%, p=0.029) and greater infarct size (15.0% [7.9-24.5] versus 18.1% [10.8-31.6], p=0.047). There was a trend of larger number of segments with more than 75% of infarct transmurality (3.0 [2.0-5.0] versus 4.0 [2.0-6.0], p=0.090) and smaller myocardial salvage index (44.5 [26.7-57.9] versus 32.2 [27.7-54.3, p=0.132) in the patients with hyperglycemia than normoglycemic patient. In multivariate analysis, hyperglycemia at admission was significantly associated with the risk of the large infarct (infarct size > median of 16.0 %) (OR 2.295, 95% CI 1.057-4.983, p=0.036) and the extensive transmurality (the number of segment with more than 75% of infarct transmurality > 5) (OR 1.008, 95% CI 1.001-1.014, p=0.022). Conclusions Admission hyperglycemia in AMI patients undergoing successfully early invasive intervention is independently associated with the large infarct size and extensive transmural infarction on CE-MRI.