For right ventricular volume determination a new method was developed that took into account right ventricular imaging by apical two-dimensional echocardiography in the four- and two-chamber views. Right ventricular volume was calculated by subtraction of the partial volume of the left ventricle and interventricular septum from the total heart volume of the right and left ventricles including the interventricular septum. Accuracy of the subtraction method was evaluated in isolated human hearts by two-dimensional echocardiography and radiography. Regression analysis indicated that for both the echocardiographic and radiologic procedures, the subtraction method gave the best correlation with the right ventricular filling volume. The correlation coefficients obtained using the echocardiographic procedure were 0.964, 0.848, and 0.851 for the subtraction method, pyramid method, and Simpson's rule method, respectively. For the radiographic procedure, these values were 0.946, 0.860, and 0.872. Percentage difference calculations indicated that the subtraction method systematically underestimated (-7.4%) the filling volume in the echocardiographic procedure, but this was not seen with the radiologic procedures. The random error associated with the subtraction method was only half (10.5%, 13.9%) of that seen with the other two methods in both the echocardiographic and radiological procedures. These results indicate that right ventricular volume determination by the subtraction method seems to be suitable for apical two-dimensional echocardiography and is more accurate than the pyramid method and Simpson's rule method.