Background: Postoperative cerebral hyperperfusion syndrome (HS) after carotid endarterectomy (CEA) causes miserable adverse cerebral complication, such as intracranial hemorrhage. Furthermore, the prognosis of patients with HS is poor. Previous studies reported that impaired cerebral vasoreactivity was implicated in HS (1). However, Powers' staging (2), one of clinical indices of cerebral hemodynamic impairment, has not been tested whether that is a factor of HS. In addition, near infrared spectroscopy (NIRS) said to reflected cerebral vasoreactivity (3). Therefore, we tested the hypothesis whether Power's staging was a factor of HS, or intraoperative NIRS measurement predicted HS. Methods: The records of 116 consecutive patients with CEA were reviewed. Logistic regression analysis was used to assess the effects of Power's staging and following factors on HS; age, sex, history of hypertension, diabetes mellitus, smoking, atrial fibrillation, symptom for internal carotid artery stenosis, history of ischemic heart disease, severity of carotid artery stenosis, stump pressure, and systemic blood pressure after declamping. Tissue oxygen index (TOI, were measured using NIRO 200 (Hamamatsu Photonics, Hamamatsu) during CEA in 5 patients preliminary. Results: Sixteen patients (18%) had HS after CEA. Significant predictors with univariate logistic regression analyses were Powers' staging, ipsilateral carotid artery stenosis (>80%), contralateral carotid artery stenosis (>50%), and systolic arterial pressure (>120mmHg) after declamping of carotid artery. Multivariate logistic regression analysis demonstrated that the Powers' staging (p=0.001), contralateral internal carotid artery stenosis more than 50% (p=0.003) and systolic arterial pressure (>120mmHg) (p=0.01) were significant predictive variables for postoperative HS. During CEA, tissue oxygen index (TOI) was measured in 5 patients of 16 patients who suffered from HS. The values of TOI after declamping of internal carotid artery were significant higher than that during clamping (61 ± 5 to 68 ± 8%, p<0.05). Conclusion: The results in our retrospective study demonstrated that the Powers' staging, as an index of hemodynamic state, contralateral carotid artery stenosis more than 50% and systolic arterial pressure (>120mmHg) after carotid artery declamping were predictive factors for postoperative HS. NIRS monitoring, TOI could be effective to predict postoperative HS as an intraoperative index.