Background: Coronary plaque of culprit lesion in patients with acute coronary syndrome is known to have features of ���vulnerable plaque��� such as higher content of lipid pool and more inflammatory reactions. The purpose of this study is to evaluate tissue composition of coronary plaques in patients with stable angina or acute myocardial infarction (AMI) using IB-IVUS (YD Ltd., Japan) and to confirm whether the IB-IVUS findings of plaques correlate to their clinical manifestations. Methods: Seventy-six significant de novo coronary lesions (stable angina n=44, AMI n=32) were evaluated with a 30 MHz phased-array IVUS catheter with motorized pull-back system at 0.5 mm/s (Galaxy II, Boston Scientific, USA) prior to percutaneous coronary intervention (PCI) with stenting. Tissue characterization of target lesions by IB-IVUS was based on radiofrequency backscatter signal analysis. Radiofrequency signals were classified into four component types: calcification, fibrotic tissue, mixed tissue, and lipid pool. For each cross section image, the content of these plaque components was assessed by an integrated soft ware. Total and mean content values for each tissue component from all the image slices for each lesion were obtained and compared between stable angina and AMI.