Background and Purpose-Recent studies have investigated plaque morphology to determine patients who are at high risk of carotid atherosclerosis. In this study, we investigated whether a difference in dynamic enhancement pattern in plaque components could be useful to assess plaque stability with multidetector CT angiography. Methods-Fifty-nine lesions with moderate to severe carotid atherosclerosis in 51 patients (33 symptomatic, 18 asymptomatic) were consecutively included. Early-and delayed-phase images were obtained in 3 equivalent axial slices with multidetector CT angiography. Hounsfield units (HU) in the early phase were subtracted from those in the delayed phase in plaques (⌬HU) and compared with clinical features, MRI-based plaque characteristics, and histological findings with 20 surgical specimens acquired from carotid endarterectomy. Results-The ⌬HU was significantly higher in asymptomatic than that in symptomatic presentation (Pϭ0.02). With MRI, a higher ⌬HU was negatively correlated with signal intensity on T1-weighted imaging (rϭϪ0.56, PϽ0.0001). Histology confirmed that ⌬HU was positively correlated with fibrous tissue (rϭ0.67, Pϭ0.001) and negatively correlated with a lipid-rich necrotic core with hemorrhage (rϭϪ0.70, PϽ0.001). Moreover, less neovascularization and inflammation was found in plaques with a higher ⌬HU. Conclusions-Delayed-phase images provide information regarding the dynamic change in contrast media from the early arterial phase. An increase in HU from the early phase on multidetector CT angiography indicates plaque stability with more fibrous tissue and a less lipid-rich necrotic core, intraplaque hemorrhage, and neovascularization. (Stroke. 2012; 43:393-398.)