The clinical and hemodynamic findings in 13 consecutive patients with "swinging heart" on M-mode echocardiography were analyzed. In these patients the anterior right ventricular and posterior left ventricular walls and interventricular septum moved almost parallel to each other throughout the cardiac cycle, often with exaggerated excursion. In 10 of 13 patients right heart catheterization revealed the hemodynamic profile of cardiac tamponade, while one additional patient was found to have evidence of cardiac compression at the time of surgery. In the remaining two patients no acute invasive diagnostic procedures were performed. During the same observation period cardiac tamponade was observed in five patients without echocardiographic evidence of a swinging heart, and four of these had large clots in the pericardial space. Thus, the swinging heart pattern appears to be a reliable marker of cardiac tamponade, except in those patients with intrapericardial lesions which mechanically limit cardiac motion. Indexing Words: Swinging heart Since 1965 echocardiography has been accepted as a useful clinical tool in the diagnosis of pericardial effusion.' Multiple criteria have been suggested for the echocardiographic diagnosis of pericardial effusion causing cardiac compression. These have included diminished ventricular volume,' phasic variations in ventricular dimens i o n ~, ~, ~ changes in mitral valve closing veloc-it^,^,^ and abnormalities of free wall Identification of tamponade, however, continues to depend on hemodynamic evaluation, part of which is obtained only by invasive procedures. The abnormal echocardiographic pattern of parallel motion of the anterior and posterior walls of the heart throughout the cardiac cycle was first From the Division of