BACKGROUND: The prognosis and treatment of patients with low-flow (LF) severe aortic stenosis are controversial. METHODS AND RESULTS: The Placement of Aortic Transcatheter Valves (PARTNER) trial randomized patients with severe aortic stenosis to medical management versus transcatheter aortic valve replacement (TAVR; inoperable cohort) and surgical aortic valve replacement versus TAVR (high-risk cohort). Among 971 patients with evaluable echocardiograms (92%), LF (stroke volume index ≤35 mL/m(2)) was observed in 530 (55%); LF and low ejection fraction (<50%) in 225 (23%); and LF, low ejection fraction, and low mean gradient (<40 mm Hg) in 147 (15%). Two-year mortality was significantly higher in patients with LF compared with those with normal stroke volume index (47% versus 34%; hazard ratio, 1.5; 95% confidence interval, 1.25-1.89; P=0.006). In the inoperable cohort, patients with LF had higher mortality than those with normal flow, but both groups improved with TAVR (46% versus 76% with LF and 38% versus 53% with normal flow; P<0.001). In the high-risk cohort, there was no difference between TAVR and surgical aortic valve replacement. In patients with paradoxical LF and low gradient (preserved ejection fraction), TAVR reduced 1-year mortality from 66% to 35% (hazard ratio, 0.38; P=0.02). LF was an independent predictor of mortality in all patient cohorts (hazard ratio, ≈1.5), whereas ejection fraction and gradient were not. CONCLUSIONS: LF is common in severe aortic stenosis and independently predicts mortality. Survival is improved with TAVR compared with medical management and similar with TAVR and surgical aortic valve replacement. A measure of flow (stroke volume index) should be included in the evaluation and therapeutic decision making of patients with severe aortic stenosis. CLINICAL TRIAL REGISTRATION: URL: http://www.clinicaltrial.gov. Unique identifier: NCT0053089.4.