To assess left atrial volume and function in aortic stenosis, 20 patients with this condition and 10 normal controls were studied. Atrial volumes were measured by echocardiography at mitral valve opening (maximal), onset of atrial systole (P wave of the electrocardiogram) and mitral valve closure (minimal), using biplane techniques. The maximal volume was greater in those patients with aortic stenosis as compared to the controls (74.8 +/- 26.4 cm3 vs. 46.4 +/- 11.9 cm3, p < 0.005), and was directly related to left ventricular mass (r = 0.77). The passive emptying volume (maximal minus onset of atrial systole) was similar in the two groups (21 +/- 8 cm3 vs. 18.7 +/- 5.9 cm3, p = NS), while active emptying volume (onset of atrial systole minus minimal) was higher in aortic stenosis (16.8 +/- 5.2 cm3 vs. 10.2 +/- 2.5 cm3, p < 0.001). The total emptying volume (sum of passive and active) was slightly higher amongst those with aortic stenosis (37.4 +/- 10.2 cm3 vs. 28.9 +/- 7.5 cm3, p < 0.05). The passive emptying fraction (passive emptying volume/maximal) was lower in the aortic stenosis group (0.28 +/- 0.08 vs. 0.40 +/- 0.05, p < 0.001), while the active emptying fraction (active emptying volume/volume at onset of atrial systole) was similar between the two groups (0.33 +/- 0.09 vs. 0.37 +/- 0.05, p = NS). Increased left atrial size in aortic stenosis is directly related to left ventricular mass and restores left atrial total emptying volume, despite the depressed passive emptying fraction. Left atrial dilatation thus represents an important compensatory mechanism, contributing to the maintenance of left ventricular stroke volume and cardiac output in severe aortic stenosis.