Background: The use of the ratio of mitral inflow and annular velocity to stroke volume (E/E���/SV) has been reported as an index of diastolic elastance (Ed). However, its change during exercise has not been evaluated previously. We hypothesized that Ed during exercise would be abnormal in patients with apical HCM (ApHCM). Methods and Results: To provide a continuous variable that estimates Ed, E/E��� was used as an estimation of mean left atrial pressure (E/E���). Stroke volume (SV) was measured from the left ventricular (LV) outflow tract diameter and TVI obtained by the pulse wave Doppler signal. Operant Ed was then estimated as E/E��� divided by the volume of filling during diastole, assuming no aortic regurgitation (SV). These parameters were measured at rest and during graded supine bicycle exercise (25W, 3 minutes increments) in 15 patients with apHCM (12 male, mean age 57 years) and 15 age and gender-matched control subjects. E/E��� at rest and during exercise was significantly higher in patients with apHCM compared with control, whereas SV at rest and during exercise did not differ significantly between the groups. Ed was not significantly differ at rest and until 25 W of exercise (0.19짹0.05 vs 0.17짹0.05 cm/s at rest, p=0.27; 0.21짹0.06 vs 0.18짹0.09 cm/s at 25 W, p=0.31). However, Ed was significantly higher at 50 W of exercise in patients with apHCM compared with that of controls (0.21짹0.05 vs 0.15짹0.04 cm/s, p=0.0059). Conclusion: Noninvasively-determined Ed was similar at rest and during mild exercise between patients with apHCM and control. However, Ed was significantly higher during moderate exercise in patients with apHCM, suggesting dynamic change of LV stiffness during exercise in patients with apHCM.