Background Risk stratification of patients with low-gradient (LG) severe aortic stenosis (AS) despite preserved left ventricular ejection fraction remains challenging. We sought to evaluate the relationship between the dimensionless index (DI)-the ratio of the left ventricular outflow tract time-velocity integral to that of the aortic valve jet-and mortality in these patients. Methods Seven hundred fifty-five patients with LG severe AS (defined by aortic valve area ≤1 cm or aortic valve area indexed to body surface area ≤0.6 cm/m and mean aortic pressure gradient <40 mm Hg) and preserved left ventricular ejection fraction ≥50% were studied. Flow status was defined according to stroke volume index <35 mL/m (low flow, LF) or ≥35 mL/m (normal flow, NF). Results After adjustment for age, sex, body mass index, Charlson comorbidity index, history of hypertension, history of atrial fibrillation, AS-related symptoms, left ventricular ejection fraction, indexed left ventricular ventricular mass, aortic valve area, and aortic valve replacement as a time-dependent covariate, patients with LG-LF and DI<0.25 exhibited a considerable increased risk of death compared with patients with LG-NF and DI≥0.25 (adjusted hazard ratio, 2.41 [95% CI, 1.61-3.62]; <0.001), LG-NF and DI<0.25 (adjusted hazard ratio, 1.84 [95% CI, 1.24-2.73]; =0.003), and LG-LF and D≥0.25 (adjusted hazard ratio, 2.27 [95% CI, 1.42-3.63]; <0.001). In contrast, patients with LG-LF and DI≥0.25, LG-NF and DI<0.25, and LG-NF and DI≥0.25 had similar outcome. DI<0.25 showed incremental prognostic value in patients with LG-LF severe AS but not in patients with LG-NF severe AS. Conclusions Among patients with LG severe AS and preserved left ventricular ejection fraction, decreased DI<0.25 is a reliable parameter in patients with LF to identify a subgroup of patients at higher risk of death who may derive benefit from aortic valve replacement.