Atheromatous plaques obtained at autopsies and surgical carotid endarterectomy specimens were scanned in a water bath with an 8 MHz high resolution real time instrument. The findings were correlated with histology at corresponding sites in each specimen. Fatty streaks were not detectable by ultrasound. Small fibro fatty plaques appeared as localized thickening of the arterial wall with little change in echogenicity or echo texture. In larger fibro fatty lesions, sizable aggregates of amorphous lipid residue appeared less echogenic than adjacent tissues and regions of dense fibrosis more echogenic. Densely calcified foci in plaques were highly echogenic and associated with acoustic shadowing. Surface irregularities of plaque at ultrasound may simulate ulceration. Ulcers, evident at gross inspection, appeared as marked surface irregularities or excavations at ultrasound. The basis of the findings and their relevance to clinical vascular ultrasound imaging are discussed.