The aortic regurgitation (AR) index, proposed as an objective indicator of postprocedural AR, decreases in proportion to AR severity, besides reportedly providing additional prognostic information. Meanwhile, left ventricular (LV) function has also been considered an essential prognostic factor. This study aimed to clarify whether LV function affected the AR index using cardiac catheterization data.A retrospective study was performed in patients whose LV function was evaluated using a micromanometer-tipped catheter. Patients with grade 2 or higher AR were excluded to explore non-AR factors affecting the AR index value. The AR index was calculated as a ratio of the gradient between the aortic diastolic blood pressure (DBP) and the LV end-diastolic pressure (EDP) to the aortic systolic blood pressure (SBP): AR Index = [ (DBP - LVEDP) / SBP] × 100.A total of 64 patients [age, 62 (interquartile range: 48-70) years; LV ejection fraction, 19% (16%-26%) ] were examined. AR index values ranged from 18.3 to 68.6. Despite having no AR, two patients displayed an AR index < 25, indicating significant AR. Multiple-regression analysis revealed that LV diastolic stiffness (β = -0.750, P < 0.001), LV max dP/dt (β = -0.296, P = 0.006), and heart rate (β = 0.284, P = 0.011) were independent determinants of the AR index value.Patients with impaired LV diastolic function and preserved systolic function had low AR index values. The additional prognostic information of the AR index may be related to LV diastolic function.