Atrial filling fraction obtained by left ventricular echocardiogram (AFF by LV echo) is considered to be a reliable measure of AFF of LV. However, in patients with LV asynergy, AFF by LV echo cannot be evaluated correctly by this method. To obtain AFF, we devised a new echocardiographic index of AFF, obtained from the aortic-left atrial echogram (AFF by Ao echo), and examined the significance of this index in 9 normal subjects (Normals) and 49 patients with various heart diseases. The correlation between AFF by Ao echo and left ventricular end-diastolic pressure (LVEDP) also was examined. In an additional 20 patients with acute myocardial infarction (acute MI), the relationship between AFF by Ao echo and pulmonary arterial end-diastolic pressure (PAEDP) was studied for several days following the onset of MI. Results were as follows: In Normal patients and patients without asynergy, a significant correlation was seen between AFF by LV echo and AFF by Ao echo (r = 0.710, p less than 0.001). The value of AFF by Ao echo was always greater than that by LV echo. AFF by Ao echo in patients with hypertensive heart disease (HHD), angina pectoris (AP) and old myocardial infarction (old MI) was significantly higher than that in Normal patients. A significant curvilinear correlation was seen between AFF by Ao echo and LVEDP (r = 0.673, p less than 0.005). In patients with acute MI, AFF by Ao echo correlated well with PAEDP.(ABSTRACT TRUNCATED AT 250 WORDS)