We have previously shown that high-frequency, high-resolution ultrasound can characterize the acoustic properties and composition of fatty plaques in cholesterol-fed rabbits. To determine whether quantitative ultrasound can delineate the regression of atherosclerotic lesions by detecting a change in their composition from fatty to fibrous types induced by alterations in dietary regimen, we fed six New Zealand White rabbits a 2% cholesterol diet for 3 months, followed by a standard diet for 3 additional months to promote the development of fibrous intlmal lesions. Segments of aortas were excised, and backscattered radiofrequency data were acquired from 400 to 600 independent sites in each specimen with an acoustic microscope operated at 50 MHz. Control data were provided by measuring backscatter from adjacent portions of the aortas devoid of lesions. Histological and immunocytochemical analyses of the fibrous intimal lesions confirmed the presence of smooth muscle cells and abundant connective tissue with little appreciable lipid. Backscatter from normal aortic segments (&#151;30.7±1.0 dB) was approximately 10-fold greater than that from fibrous lesions (-42.4±1.0 dB;p<.05). We previously reported that integrated backscatter from fatty lesions was -50.6±0.7 dB, or approximately 10-fold less than that from fibrous lesions (p<.05). Values for integrated backscatter from the media of each tissue type were approximately equal (&#151;30.0±1.7 versus &#151;30.7±1.6 versus &#151; 33.4±0.8 dB for normal versus fibrous versus fatty tissues, respectively;P=notsignificant). Thus, quantitative ultrasonic tissue characterization sensitively differentiates between fibrous and fatty atherosclerotic plaque and normal tissue in the presence of experimental diet-induced atherosclerosis. These results suggest that alterations in dietary regimens may elicit changes in the composition of vascular plaques that are detectable with quantitative ultrasound.