To correlate the ultrasound and MR imaging of carotid atherosclerotic plaque with its histologic compositions,20 patients(22 lesions) scheduled for carotid endarterectomy were examined by B mode ultrasound and MRI preoperatively. The specimens were also examined postoperatively and the images of carotid plaque were recorded. The transverse pathological sections of plaques were made and matched with images of ultrasound and MRI. 256 gray scale ultrasound densitometric analysis of carotid plaque compositions was performed. Ultrasonic density of calcium, fibrous tissue, hemorrage/thrombus and lipid deposits was (89?12), (53?8), (37?6) and (39?3) respectively in vivo , and was (168?11), (136?12), (85?12) and (89?10) respectively in vitro . In vivo relative signal intensities of calcium, fibrous tissue, old hemorrhage/thrombus and lipid deposits were low, equal, equal and very high respectively on T 1 W MR image, and very low, equal, high and high respectively on PDW MR image, and low, equal or slightly high, high and high respectively on T 2 W MR image. Signal intensities of plaque contents ex vivo were correlated to signal intensities in vivo. Densitometric analysis of ultrasound images of carotid plaques can quantify possible components of plaque, and MRI has the feasibility to identify the components of plaque.