The purpose of this study was to evaluate left ventricular (LV) structure-function interplay in aortic stenosis. LV structure was assessed from endomyocardial biopsies obtained in 27 patients with aortic stenosis. Total collagen volume fraction, orthogonal collagen fiber meshwork (cross-hatching) and endocardial fibrosis were determined by morphologic-morphometric evaluation. Control biopsy data were obtained from 6 pre-transplantation donor hearts whereas other 11 patients with normal cardiac function served as hemodynamic controls. LV biplane cineangiography and high-fidelity LV pressure measurement were carried out in all patients. Systolic function was assessed by LV biplane ejection fraction, diastolic function by time constant of relaxation, peak filling rates and passive elastic properties. Total collagen volume fraction (7.3 versus 1.6%, p < 0.01) as well as the degree of cross-hatching (1.7 versus 0.8 grade, p < 0.01) were significantly increased in patients with aortic stenosis with respect to controls. Endocardial fibrosis was present in 11/27 patients with aortic stenosis and in no patients of control group. In aortic stenosis in presence of increased total collagen volume fraction there were no changes in systolic and diastolic function, whereas in presence of changes in collagen architecture ejection fraction was depressed and passive elastic properties increased. In conclusion, in aortic stenosis, changes in collagen architecture are associated with altered systolic function and passive diastolic properties. The sole increase in total collagen volume fraction without a change in architecture leaves systolic and passive diastolic function unaltered. A prolongation of relaxation was present in aortic stenosis and appears to be mediated by muscle hypertrophy per se.