Background: Tissue characterization of coronary plaques is possible using IB-IVUS and higher lipid content in the target lesion has been reported to associate with acute coronary syndrome (ACS) and no reflow phenomenon during percutaneous coronary intervention (PCI). We sought to identify IVUS parameters that correlate with plaque composition assessed by IB-IVUS. Methods: Ninety-five consecutive patients (male 73%, age 59.2짹9.7) with single target lesion underwent IVUS and IB-IVUS prior to PCI. Study population included 22 patients with ACS (AMI n=14), 15 patients with diabetes and 35 patients hypercholesterolemia. Results: Relative lipid content was higher in ACS related plaques (40.9%짹11.9% vs. 29.7%짹14.0%, p=0.001), but showed no difference between diabetic and non-diabetic patients. In univariate analysis, remodeling index(r=0.292, p=0.006), plaque burden(%) (r=0.422, p=0.0001), and vessel area(mm2) of target lesion (r=0.539, p<0.0001) showed a significant correlation with its relative lipid content(%). In multivariate regression analysis, however, vessel area of target lesion was the only independent predictor of lipid content (%)(R2=0.364, standardized coefficients = 0.874 p=0.002). Conclusions: ACS, positive remodeling and larger plaque burden were associated with higher lipid content. However, vessel area was the only significant independent factor predicting lipid content on IB-IVUS. Therefore, overestimation of lipid content by IB-IVUS especially in large vessels cannot be ruled out. IB-IVUS data need to be validated in further clinical trials.