Left ventricular microcatheterization at the bedside was accomplished in 20 acutely ill patients with myocardial infarction. Direct measure of the left ventricular end-diastolic pressure shows that its values can substantially differ from those of the pulmonary artery end-diastolic pressure. The authors observed that left ventricular microcatheterization is indicated, in the course of acute myocardial infarction, in the following circumstances: --when either the pulmonary artery end-diastolic or the pulmonary capillary pressure is beyond normal limits; --in the presence of tachyarrhythmias which, for alteration of the dynamics of atrial contraction, increase the difference between the left ventricular and pulmonary artery end-diastolic pressures; --when a mitralic defect is associated with the acute myocardial infarction.