Precise information concerning the behavior of the heart in pneumonia is essential for the rational management of the circulation in this disease. Whether, in the course of acute pneumonitis, the cardiovascular system is functionally impaired, is still a matter of controversy. Direct methods for the estimation of myocardial function are still wanting. But alterations in the size of the heart furnish evidence, although indirect, of changes in the physical state of the myocardium, and in particular of variations in the length of heart muscle fiber. Such changes, as will be pointed out, have been shown by investigation to be associated with alterations in the volume output of the ventricles; and on the maintenance of ventricular volume output depends, in large measure, the efficiency of the circulation as a whole. The present investigation was designed to obtain, by the roentgenographic method, at relatively frequent intervals, accurate records of the size of