Purpose : Chronic hyperglycemic state might be related with degree of microvascular damage, regardless of the glucose level at the early stage of acute myocardial infarction (AMI). However, there are few data about effect of chronic hyperglycemia on the microvascular damage in AMI patients with diabetes mellitus (DM). This study was to evaluate the relationship between chronic hyperglycemic state and degree of microvascular damage in AMI patients with DM. Methods : We collected clinical, biochemical and angiographic information in 55 consecutive patients ( 44 male; age 56 짹 11 years) treated with percutaneous coronary intervention (PCI) for AMI. We measured glycosylated hemoglobin (HbA1c) and glucose level on the admission day. After PCI, we assessed coronary flow reserve (CFR), microvascular resistance index (MVRI) defined as the mean coronary arterial pressure divided by the hyperemic average peak velocity with intracoronary Doppler wire. Also, we assessed TIMI myocardial perfusion grade (TMPG) and the patients were divided into 3 groups according to the TMPG (TMPG 0/1; n = 6, TMPG 2; n = 22, TMPG 3; n = 27). Results : There was a significant correlation between CFR and HbA1c level (r = -0.30, p = 0.033). MVRI had a good correlation with HbA1c level (r = 0.41, p = 0.006) and glucose level (r = 0.34, p = 0.03), respectively. There was also significant correlation between TMPG and HbA1c level (r = -0.464, p = 0.001) and the patients with high TMPG had low HbA1c level (TMPG 0/1; 9.50 짹 2.26 % vs TMPG 2; 7.96 짹 1.36 % vs TMPG 3; 7.22 짹 0.99 %, p = 0.002, respectively). Conclusions : At the early stage of AMI, chronic hyperglycemic state assessed by HbA1c level was associated with severe microvascular damage in AMI patient with DM. Therefore, intensive glucose control might be required to preserve microvascular integrity of infarct related myocardium in AMI patients.