Six patients with acute myocardial infarction presented with hemodynamic evidence of predominant right ventricular failure, characterized by a mean right atrial pressure averaging 20.2 mm Hg and left ventricular filling pressure averaging 16.3 mm Hg. Autopsy in two cases revealed extensive involvement of the right as well as the left ventricle. Clinically the patients usually had evidence of diaphragmatic wall infarction, distended neck veins, hypotension and heart block. Pressure contours and mean pressure often showed no significant change as the catheter was advanced from the right atrium to the pulmonary artery. Shock in three patients was effectively treated with plasma volume expansion, to increase further right-sided pressure, or the administration of sodium nitroprusside, to reduce left-sided filling pressure. It is suggested that when right ventricular infarction accompanies left ventricular infarction, a unique clinical and hemodynamic syndrome occurs because the ability of the right ventricle to maintain adequate left ventricular filling is impaired. Recognition of this syndrome is vital if appropriate therapy is to be instituted.