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We report a study showing that two-dimensional echocardiography is capable of visualizing a ventricular septal defect as small as 3 mm. The physical parameters which determine the visibility of an aperture in the myocardium include the presence of anterior refractile tissue, the amount of ultrasound beam focusing and range over which this occurs, the angle of incidence of the ultrasound beam on the septum, and the number of lines of information per degree of sector sweep. These factors are illustrated by in vitro tests. The influence of operator controllable variables in two-dimensional sector scanning echocardiography is discussed.