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Background and purpose: Fractional flow reserve (FFR) has exceled the degree of stenosis and become the new guiding post for identifying functionally severe coronary artery lesions. In the current study, we utilized a similar indicator, fractional flow (FF), to gauge the hemodynamic impact of symptomatic intracranial atherosclerotic stenosis (ICAS), and to clarify its relationships with the severity of stenosis and collateral status.Methods: Patients with symptomatic ICAS (70-99% stenosis) confirmed on digital subtraction angiography (DSA) were consecutively recruited. FF was obtained during DSA examination, defined as the ratio of pressures measured distal to an ICAS lesion (Pd) and within the aorta (Pa), by pressure sensors. Leptomeningeal collaterals was grading from zero (absent) to four (complete compensatory) based on DSA examination. The correlations between FF, anatomical stenosis and collateral status were analyzed.Results: Twenty five patients with a mean age of 55.6years were included. The median percentage of stenosis and median FF were 82.3% and 0.68, respectively. Eleven patients were evaluated as poor collaterals (grade 0-2), and fourteen patients were good collaterals (grade 3-4). Overall, the hemodynamic impact of the lesions aggravated (with a decreased FF) as the percentage of stenosis increased (r= -0.398,P =0.06). However, the collateral status significantly altered such correlation, which was more significant in patients with poor collateral circulation (r=-0.677, P=0.032), but did not exist in patients with good collateral circulation (r=-0.279, P=0.356).Conclusions: An anatomically severe (70-99%) symptomatic ICAS lesion may lay significant hemodynamic stress downstream as assessed by the indicator FF. However, a good collateral circulation may mitigate such hemodynamic impact, which may partly explain the protective effect of good collaterals against recurrent stroke in such patients.