BACKGROUND: Grading of severe (aortic valve area ≤1 cm) aortic stenosis with preserved left ventricular ejection fraction is based on a classification depending on flow (normal flow versus low flow) and pressure gradient (low gradient versus high gradient). The aim of the present study was to compare the outcome of patients with normal flow high gradient and low flow high gradient severe aortic stenosis (SAS) with no or minimal symptoms. METHODS: This multicenter study enrolled 983 consecutive patients (mean age 75±11 years, 459 women) with asymptomatic or minimally symptomatic HG (mean pressure gradient ≥40 mm Hg) SAS with preserved left ventricular ejection fraction. Low flow was defined by Doppler echocardiography as a stroke volume index <30 mL/m (n=131) or a stroke volume <55 mL (n=136). The end point was all-cause mortality. RESULTS: During a median follow-up period of 48 (45-52) months, 225 patients (23%) died. The 60-month mortality was higher in low flow high gradient SAS compared with normal flow high gradient SAS (36±5% versus 22±2% and 38±5% versus 21±2% for stroke volume index and stroke volume, respectively, both <0.0001). After adjustment for outcome predictors including aortic valve replacement as time-dependent covariate, low flow high gradient SAS displayed considerable mortality risk during follow up compared with normal flow high gradient SAS (adjusted HR 2.17 [1.51-3.13]; <0.0001 for stroke volume index <30 mL/m and adjusted HR 1.86 [1.29-2.68]; =0.001, for stroke volume <55 mL). The prognostic impact of low flow was consistent in subgroups of patients. CONCLUSIONS: Asymptomatic or minimally symptomatic patients with low flow high gradient SAS and preserved left ventricular ejection fraction have a considerable increased risk of mortality during follow-up. These patients should be promptly considered for aortic valve replacement.