Background: The utility of CT perfusion in evaluation of cerebral hemodynamic state in severe carotid artery stenosis has not been well documented. The purpose of this study is to describe the perfusion findings of patients with symptomatic carotid occlusive disease using CT perfusion and some factors in having effects on impaired hemodynamics. Methods: We analyzed 12 patients with severe carotid disease who underwent CT perfusion and cerebral angiography. We also evaluated the collateral supply to the territory ipsilateral to the ICA stenosis from angiographic findings. Results: 3 patients had normal TTP values and the other 9 patients had delayed TTP. The perfusion values of CBF and CBV maps were variable. The degree (>90% of carotid stenosis or carotid occlusion with contralateral carotid stenosis) of stenosis and collateral supply were important factors that decided hemodynamic states of severe ICA disease. Conclusions: TTP may be sensitive maps for the identification of hemodynamic compromise and cerebral perfusion may be influenced by the state of collaterals and other related artery.