We have measured pulmonary artery (PA) and left ventricular diastolic pressures (LVDP) in patients with acute myocardial infarction to establish the relationships of PA pressure to LVDP. Paired determinations for the various parameters showed (mean difference in mm Hg): left ventricular end-diastolic pressure (LVEDP)-LVDP pre-a + 7.9,P< 0.001; LVEDP-mean PA wedge + 6.0,P< 0.001; mean PA wedge-LVDP pre-a, + 0.8,P> 0.2; PA end-diastolic pressure (PAEDP)-mean PA wedge (in all patients) +3.3,P< 0.001; PAEDP-mean PA wedge (patients with pulmonary vascular resistance ≦2 units) +1.3,P< 0.1; LVEDP-PAEDP +4.7,P< 0.001; and LVEDP-mean PA −2.0,P< 0.02. The relationship of LVEDP to mean PA wedge was: LVEDP (y) = 1.12 mean PA wedge (x) +4.69; Sy.x = 3.42; r = 0.92.After acute myocardial infarction, PA pressures did not accurately reflect LVEDP because atrial contraction made a large contribution to ventricular filling pressure. In addition, PAEDPs were not the same as mean PA wedge pressures because of some increase of pulmonary vascular resistance in many patients. Thus, PA pressures only provided reliable information about the level of pulmonary venous pressure. LVDP pre-a correlated well with mean PA wedge pressure, and therefore measurement of LVDP (pre-a and EDP) yielded information not only about pulmonary edema, but also about LV performance.