['The degree of carotid artery stenosis has traditionally been used as a marker of hemodynamic compromise and increased stroke risk. However, the hemodynamic effect of carotid atherosclerotic plaque length on cerebral blood flow has not previously been studied.', 'To determine whether carotid plaque length, in addition to degree of stenosis, significantly affects carotid blood flow in patients with >65% carotid stenosis.', 'Consecutively treated surgical patients with unilateral >65% carotid stenosis at a single institution were analyzed. Quantitative measurements of plaque length, internal carotid artery (ICA) vessel diameter, and degree of stenosis were made from magnetic resonance angiography images. Quantitative phase-contrast magnetic resonance angiography flow maps were generated to estimate ICA flow compromise by calculating a ratio of the ipsilateral/contralateral ICA flow rates.', 'Of 38 eligible patients, 23 had full anatomic and ICA flow data sets available for analysis. Univariate regression analysis demonstrated that longer carotid plaques and increasing percentage carotid stenosis were associated with a significant decline in ipsilateral ICA flow (P = .008 and P = .02, respectively). A multivariate regression identified both plaque length and vessel diameter as independent predictors of ICA flow (P = .001 and P = .002, respectively).'] Do extracranial carotid plaque length and parent vessel diameter significantly affect baseline ipsilateral intracranial blood flow?