Ultrasound echocardiograms from the septal and posterior left ventricular walls were displayed with a simultaneously recorded electrocardiogram, phonocardiogram, and indirect carotid pulse. These echoes differed in both amplitude and waveform. The contour of the posterior wall echo resembled an inverted ventricular volume curve, while the septal echo was of smaller amplitude and had a characteristic notched appearance. Most of the movement of the left ventricular walls relative to the ultrasound transducer was attributable to systolic contraction and diastolic expansion of the cavity. However, superimposed on this motion due to change in cavity size was movement of the left ventricle as a whole, first anteriorly toward the ultrasound transducer during late systole then posteriorly away from it at the beginning of left ventricular relaxation. These movements added to the amplitude of posterior wall motion but subtracted from the motion of the septum and were responsible for the notch in the waveform of this echo. Attachment superiorly to the aortic root might also have limited septal motion which was less near the base than nearer the apex of the left ventricle.The internal left ventricular dimension measured by ultrasound was standardized by using the mitral valve as a landmark and by recording the motion of the left side of the interventricular septum and endocardial surface of the posterior left ventricular wall simultaneously. This measurement was reproducible. In normal subjects, the ultrasonic dimension measured 4.40 ± 0.28 cm at the beginning of systole and shortened by 35.5 ± 3.9% at a rate of 1.22 ± 0.31 lengths/sec. By contrast, the average figures for six patients with primary myocardial disease were 6.96 ± 0.43 cm, 14.9 ± 4.2%, and 0.64 ± 0.11 lengths/sec. Calculation of such indices of left ventricular size and of rate and extent of myocardial shortening should be useful in the detection of impaired myocardial function and in following its progress.