In a consecutive series of 1,225 routine echocardiograms, a pericardial effusion (PE) was present in 15% and was considered substantial in 10%. Sixty-one percent (68/111) of the latter were clinically unsuspected. A retrospective analysis of these patients showed that 60% (41/68) had underlying heart disease such as congestive heart failure, left ventricular hypertrophy, or asymmetric septal hypertrophy without other discernible causes for PE. In patients referred specifically for suspected pericardial effusion, 67% (29/43) had diseases recognized as causes of pericardial effusion. Unsuspected substantial PE are commonly detected by echocardiography, especially in patients with underlying heart disease.