Acute hyperglycemia is associated with increased mortality rates in patients with acute coronary syndrome. This study aimed to evaluate the relationship between the glucose level and clinical variables during primary intervention in patients with ST-elevation acute myocardial infarction (STEMI). Of consecutive 94 patients with STEMI treated by primary stent implantation, acute hyperglycemia (plasma glucose level on admission>198 mg/dl) was recognized in 29 patients. There were no significant differences in baseline characteristics, except for the presence of diabetes and HbA(1c) level, between patients with and without acute hyperglycemia. In patients with acute hyperglycemia, corrected TIMI frame counts were significantly higher compared with those in patients without acute hyperglycemia (46.3+/-30.3 vs. 34.0+/-17.9, p=0.02). And corrected TIMI frame count was independently associated with plasma glucose level (p=0.006). Maximum level of creatine kinase (CK) and CK-MB were significantly higher in patients with acute hyperglycemia (CK, 4840.0+/-4690.3 vs. 2410.7+/-2302.9 IU, p=0.001; CK-MB, 315.3+/-257.7 vs. 195.9+/-191.1, p=0.01). The presence of acute hyperglycemia was associated with the impairment of epicardial coronary flow after primary stent implantation. This mechanism might be responsible for the increased infarct size.