The purpose of this study was to investigate the echocardiographic effects of absence of the pericardium. Five patients with congenital complete absence of the left pericardium were studied. All had typical chest X-rays, four had cardiac catheterization which excluded any intracardiac shunts, and one had diagnostic pneumothorax. All five had an enlarged right ventricular dimension (RVD): 1.9 +/- 0.1 cm/m2 (normal: less than 1.3 cm/m2) and abnormal interventricular septal (IVS) motion (three Type A, two Type B). Sixteen additional patients were studied after pericardial stripping for a variety of conditions. In none was cardiopulmonary bypass used. Eight of these patients had preoperative echocardiograms; all showed normal IVS motion. After surgery RVD was large in all 16 patients, increasing from 1.0 +/- 0.2 cm/m2 preoperatively to 1.7 +/- 0.1 cm/m2 postoperatively, P less than 0.01. Fourteen of the 16 patients had abnormal IVS motion, nine Type A, and five Type B. We conclude that absence of the pericardium results in echocardiographic abnormalities which mimic those seen in right ventricular volume overload. This may be due to altered cardiac position and motion within the thorax resulting from loss of normal pericardial restraint.