To determine the feasibility of quantitative ultrasonic techniques to define the composition of athero sclerotic plaques, samples of freshly excised human aortas were sewn to a sample holder and immersed in a saline bath for ultrasonic interrogation. Integrated backscatter of a 2-microsecond portion of the backscattered radiofrequency signal was measured with a 10 MHz focused transducer. Integrated backscatter was calculated by normalizing the root-mean-square voltage of the gated signal to the root-mean-square voltage obtained by replacing the tissue sample with a nearly perfect reflector. Microscopic examination of the 124 interrogated sites allowed differentiation of normal, fibrous, fibrofatty, and calcified regions. A site was considered calcified if it contained any histochemically detectable calcium. Values of integrated backscatter were markedly elevated from calcified regions (−30.0 ± 6.4 dB, n = 25; mean ± SD; p< 0.001) compared to normal (−43.2 ± 2.4 dB, n = 20) and fibrofatty (− 43.9 ± 3.4 dB, n = 43) sites. Values from fibrous regions (− 40.7 ± 3.8 dB, n = 36) were also significantly different compared with the calcified and fibrofatty regions (p<0.001). Thus, we have demonstrated that quantitative ultrasonic backscatter identifies and differentiates calcification and fibrosis in atherosclerotic sites offering promise for the noninvasive assessment of the pathologic status of the arterial wall.