Hyperperfusion syndrome (HPS) is a rare but potentially fatal postoperative complication after revascularization, while preoperative predictor of HPS had not been fully established. We used pseudo continuous arterial spin labeling (pCASL) to investigate the correlation between the collateral flow proportion and the elevated cerebral blood flow (CBF) ratio relative to the preoperative CBF in patients with unilateral internal carotid artery (ICA) stenosis. A significant correlation was observed between the collateral flow proportion and the elevated CBF ratio . (r =0.588, P =0 .01). As an indication, HPS are likely to occur in patients with low preoperative CBF and good collateralization.