Using echocardiography, various indices of left ventricular function were studied in a group of 11 patients with small posterior pericardial effusion. The only index of left ventricular function that was abnormal in this group was maximum diastolic endocardial velocity (DEVM), a measure of left ventricular relaxation. When evaluated as a group, DEVM was significantly decreased in the presence of effusion as compared with the control group. This change in DEVM in the presence of small effusion, without any other associated changes in left ventricular function, suggests that the mechanism of impaired left ventricular relaxation is a change in compliance of the pericardium. In large effusions, left ventricular relaxation abnormalities are associated with changes in left ventricular dimensions, and impaired relaxation may also be the result of shift in intracardiac structures.