Simultaneous measurements of left ventricular dimension and wall thickness by echocardiography, and of pressure by micromanometer, were made at cardiac catheterization in 30 patients with heart disease, in order to study mechanisms of impairment of left ventricular systolic function. Echocardiograms and pressure traces were digitized so that continuous measurements of left ventricular wall thickness and dimensions with their rates of change could be obtained. Ejection fraction was estimated from digitized cineangiograms. In all patients, except those with severe mitral regurgitation, there was close correlation (r=0.92) between peak left ventricular dP/dt and peak rate of reduction of dimension. Myocardial power values, calculated as the product of circumferential shortening rates and wall stress were plotted throughout the cardiac cycle, and peak values in patients with normal left ventricular function were in the range 30 to 60 mW cm-3 myocardium. Pressure-dimension loops were constructed, which reflected the relation between the function of a localized region of cavity studied by echocardiography and that of the ventricle as a whole in the pressure wave form. Incoordinate contraction was associated with distortion of the loop and a reduction in its area to less than 75% that of the maximum for the cycle in question (cycle efficiency). In patients with left ventricular disease, ejection fraction, peak power, and cycle efficiency were all reduced, either singly or in combination. There was no consistent pattern, however, suggesting that clinical left ventricular disease may be the resultant of a number of different types of disturbance. These include structural abnormalities, reduction in peak rates of myocardial shortening or power development, and incoordinate contraction. The present investigation suggests ways in which these may be separated and studied in individual patients.