The purpose of this study was to determine if respiratory variation and/or absolute size of echocardiographically measured right or left ventricular internal dimension at end-diastole (RVIDd or LVIDd) are accurate indicators of the presence or severity of cardiac tamponade. We measured RVIDd or LVIDd by echocardiography in nine closed-chest, spontaneously breathing dogs in control and during hypotensive tamponade. With tamponade, the end-expiratory RVIDd and LVIDd were significantly smaller than control. Inspiratory increases in RVIDd and decreases in LVIDd were exaggerated during tamponade. Because of the wide range and overlap of RVIDd and LVIDd, no single expiratory value or amount of respiratory change indicated the presence or severity of tamponade. We conclude that if serial echocardiograms show a pericardial effusion, a decreasing end-expiratory RVIDd and LVIDd and an increasing percentage change in ventricular diameter with inspiration, a progressive degree of tamponade should be suspected. However, a single echocardiogram cannot accurately predict the presence or severity of tamponade.