Background: Multi-detector row computed tomography (MDCT) has shown to be useful as noninvasive assessment for coronary artery disease. However, the ability of MDCT to differentiate plaque types remains unclear yet. We sought to investigate whether density of plaque by MDCT correlates with plaque composition assessed with integrated backscatter intravascular ultrasound (IB-IVUS). Method: Eighty-eight consecutive patients were investigated by MDCT before scheduled coronary angiography. One hundred and eight lesions in 88 patients were evaluated with IVUS and IB-IVUS when the lesion was suspected to have more than 50% stenosis angiographically. Based on IB-IVUS analysis, tissue components were classified into calcified, dense fibrous, fibrous tissue and lipid core and automatically quantified by software. The CT density (measured as HU) within plaque was measured with a centrally placed region of interest at the corresponding location in IB-IVUS. Results: 106 lesions showed atherosclerotic plaque with >50% stenosis among 108 lesions evaluated by IB-IVUS. Plaque was identified in 89 lesions by MDCT (sensitivity 84%). In only 1 lesion diagnosed to have atherosclerotic plaque with MDCT, IB-IVUS showed normal coronary artery (positive predictive value 99%). The mean relative content of calcified tissue, dense fibrous tissue, fibrous tissue, and lipid core by IB-IVUS was 2.10짹2.16%, 5.90짹6.21%, 47.93짹14.34%, and 36.40짹12.37%, respectively. There was a significant positive correlation between the CT density and the calcified tissue content in percent (r=0.416, p<0.001). However, the density of plaques on MDCT did not show any correlation with other tissue content. Conclusion: The plaque density measured by MDCT was helpful to identify calcified atherosclerotic plaque, but could not differentiate plaque types of noncalcified lesion.