Background: Tissue characterization of coronary plaques is feasible using integrated backscatter intravascular ultrasound (IB-IVUS), and higher lipid content has been found in the target lesions of acute coronary syndrome (ACS). The present study was performed to identify clinical and IVUS parameters that correlate with plaque composition assessed by IB-IVUS. Methods and Results: A total of 109 patients (age 60.0±9.7 years) were evaluated with IVUS and IB-IVUS prior to percutaneous coronary intervention. Patients with ACS had a larger vessel size and higher plaque burden in the target lesion than those with stable angina. Relative lipid content of the target lesion by IB-IVUS was also higher in ACS (43.6%±12.0% vs 29.9%±14.2%; P<0.001). The remodeling index (r=0.403, P<0.001), plaque burden (%) (r=0.495, P<0.001), and vessel size (r=0.572, P<0.001) significantly correlated with lipid content. In the multiple regression analysis, vessel size was the most important independent predictor of lipid content followed by presence of ACS and the remodeling index. Conclusions: ACS, positive remodeling, and larger plaque burden were associated with higher lipid content of coronary plaque. However, the lipid content on IB-IVUS was also significantly affected by vessel size. Therefore, qualitative, morphologic assessment of coronary plaque rather than simple quantitative analysis of tissue components seems to be more appropriate for the identification of vulnerable plaque using IB-IVUS. (Circ J 2010; 74: 754-759)