Cardiac tamponade has been associated with an abnormally increased respiratory variation in transvalvular blood flow velocities. To determine whether this finding is consistently present in cardiac tamponade, seven patients were studied prospectively with Doppler echocardiography before and after pericardiocentesis and the results were compared with those found in 20 normal adults and 14 asymptomatic patients with pericardial effusion who did not have definite clinical evidence of tamponade. Doppler ultrasound evaluation included measurement of mitral, tricuspid, aortic, pulmonary and central venous flow velocities, as well as left ventricular ejection and isovolumic relaxation times during inspiration, expiration and apnea. In the patients with severe cardiac tamponade, respiratory variation in transvalvular flow velocities and left ventricular ejection and isovolumic relaxation times were markedly increased compared with values in normal subjects and those obtained after pericardiocentesis. In the 14 asymptomatic patients with pericardial effusion but without overt tamponade, 7 showed respiratory variation in flow velocity similar to that of normal subjects. The other seven patients demonstrated increased respiratory change compared with normal, but less than that in the patients with tamponade. Clinical and hemodynamic data in this latter group suggest that these patients may represent an intermediate stage of pericardial effusion with an element of hemodynamic compromise.