Ultrasonic tissue characterization of the myocardium has become very important because of increased need to evaluate local cardiac function. In the present study, acoustic properties of the tissue elements in myocardial infarction and dilated cardiomyopathy were measured by a scanning acoustic microscope (SAM) system developed for this purpose, and the elastic bulk modulus of the normal and pathologic myocardium was assessed from data obtained with it. The cases of acute myocardial infarction (AMI) and five cases of dilated cardiomyopathy (DCM) were studied. Autopsied myocardial tissue was fixed in formalin, embedded in paraffin, cut into slices 10 μm thick, and mounted on glass slides for SAM measurement. The SAM system was operated 100 to 200 MHz and provided two-dimensional distribution of the attenuation constant and sound speed. Values of the attenuation constant and sound speed in the region of interest were measured by comparing optical microscopic and acoustic images. Four kinds of tissue elements-normal myocardium, degenerated myocardium, granulation, and fibrosis-were observed in the AMI specimens. Three kinds of tissue elements were observed in the DCM specimens: normal and degenerated myocardium and fibrosis. Density of each tissue element was measured using the CuSCU4 method, and the elastic bulk modulus was calculated using the measured sound speed and the density of each tissue element. The attenuation constant of the AMI specimens was 0.94±0.04 dB/mm/MHz in the normal myocardium; sound speed was 1620.2±8.2 m/s. Respective values were 0.71±0.12 dB/mm/MHz and 1572.4±10.6 m/s in degenerated myocardium, 0.88±0.47 dB/mm/MHz and 1590.2±32.5 m/s in granulation, and 1.75±0.11 dB/mm/MHz and 1690.3±9.1 m/s in fibrosis. In the DCM specimen, these values were 0.92±0.04 dB/mm/MHz and 1618.4±7.4 m/s in normal myocardium, 0.69±0.11 dB/mm/MHz and 1580.2±10.2 m/s in the degenerated myocardium, and 0.70±0.09 dB/mm/MHz and 1616±10.4 m/s in fibrosis. The bulk modulus observed in the AMI specimens was 2.84×109 N/m2 in the normal myocardium. In fibrosis, the bulk modulus was quite different: 3.12×109 N/m2 in AMI and 2.80×109 N/m2 in DCM specimens. Fibrosis in both AMI and DCM contained the same chemical component, collagenous fiber; however, the structure of the fibrosis was different. The direction of fibrosis was parallel to the long axis of the myocyte in AMI, while its orientation was random in DCM. The elasticity of fibrosis was thought to have adapted to the pathophysiology of AMI and DCM. In AMI, fibrosis is known to prevent infarct expansion and cardiac rupture. In DCM, fibrosis may reduce diastolic dysfunction. The results suggest that measurement of acoustic properties is important as basic data for characterizating ultrasonic tissue of the myocardium.