The relation between quantitative ultrastructural changes of the left ventricular (LV) myocar-dium and contractile function was studied in patients with chronic aortic stenosis (AS). The volume fractions of myofibrils, sarcoplasm and mitochondria in myocardial cells were determined by electron microscopic morphometry in small LV tissue samples of 19 patients withAS. Interstitial fibrosis was measured by light microscopic morphometry. Transmural biopsies of the LV freewall perfused by the anterior descending branch of the left anterior descending coronary artery (LAD) were obtained during aortic valvereplacement. LV function was analyzed from preoperative right-and left-heart catheterization and angiography. Group 1 consisted of seven patients with ejection fractions (EFs) greater than 55% and mean left atrial pressure (LAP) less than 15 mm Hg. Group 2 consisted of 12 patients with EFs less than 55% and mean LAP greater than 15 mm Hg. Patients in group 1 had lower LV end-diastolic volume (91.9 vs 145.3 ml/m2, p< 0.05) and lower LV muscle mass (148.3 vs 199.8 g/m2, p< 0.05) than patients in group 2. The volume fraction of myofibrils was higher in group 1 than in group 2 (48.4 vs 42.1%, p< 0.05), while volume fractions of sarcoplasm (31.7 vs 36.0%) and mitochondria (20.9 vs 22.0%) were comparable (p> 0.05). Interstitial myocardial fibrosis did not differ between groups (16.3 vs 14.7%, p> 0.05). Biopsies from the area perfused by the LAD in 10 additional surgical patients who had coronary artery disease with moderate LAD stenosis and normal wall motion in the area of LV free wall perfused by the LAD were taken as controls for morphometric data. No significant difference of ultrastructural data was found between group 1 and controls. The volume fraction of myofibrils was lower in group 2 than in controls (42.1 vs 52.9%, p< 0.001), and the volume fraction of sarcoplasm was higher (36.0 vs 21.1%, p< 0.001). Mitochondria and interstitial fibrosis did not differ in group 2 and controls (p> 0.05).