Background and Objectives: Because thrombus burden, catecholamine stimulation and inflammatory substances in the myocardial infarction (AMI) can cause generalized and localized vasoconstriction, stents placed during percutaneous coronary intervention (PCI) in patients with AMI may be undersized, and associated with stent malapposition and a stent thrombosis. The aim of the study was to evaluate the changes of reference vessel diameter (RVD) of the infarct-related artery (IRA) before (Pre-PCI), immediate after PCI (post-PCI), and at follow-up by Quantitative Coronary Analysis (QCA). Methods: We studied 491 AMI (STEMI 62%) patients (353 men, 65 짹 12 years) who had a complete QCA data in pre-PCI and immediate post-PCI, and 215 patients who underwent follow-up coronary angiograghy at mean 308 days after index PCI. Using the paired t-test, we compared the changes in the RVD of IRA at pre-PCI, immediate post-PCI, and 1-year follow-up. Results: The RVD was increased from pre-PCI (3.04짹0.44mm) to post-PCI (3.18짹0.43mm; p<0.0001), and decreased from post-PCI to follow-up (3.17짹.045mm; p<0.0001). However, the RVD at before PCI was still smaller than the RVD at follow-up in significance (p<0.0001). These findings showed similar tendency with the result of each coronary arteries (Figure 1). Conclusion: RVD changed significantly from pre-PCI to immediate post-PCI and follow-up in patients with AMI. Especially, RVD of IRA in pre-PCI was smaller than that in post-PCI and follow-up. Therefore, when operators choose a stent or balloon catheter, they should consider that actual RVD may become enlarged as time goes on.