Left ventricular filling pressure has been used as a clinical guide to left ventricular function in patients with acute myocardial infarction. However, the contribution of spontaneous or induced changes in arterial pressure to left ventricular filling pressure has not been emphasized. In 26 patients left ventricular filling pressure and arterial pressure were monitored simultaneously for 1 to 5 days after acute myocardial infarction. Ventricular filling pressure averaged 20.7 mm Hg on day 1 and decreased to 12.7 by day 5. Systolic arterial pressure decreased from 146.6 to 107.4 mm Hg during this same period. These changes were significant for treated (P < 0.001) as well as untreated patients (P < 0.01). Fluctuations in arterial pressure occurring spontaneously or induced by isometric exercise, sublingual administration of nitroglycerin or infusion of nitroprusside were accompanied by similar changes in left ventricular filling pressure. These data indicate that a change in arterial pressure may be the most important determinant of a change in left ventricular filling pressure during the first few days after acute myocardial infarction.