Objective: To evaluate the diagnostic value of CT perfusion (CTP) for posterior circulation cerebral ischemia and hyperacute phase of cerebral infarction. Methods: CTP was performed in 184 patients with suspected posterior circulation acute ischemic stroke, and diffusion weighted imaging (DWI) of MRI was performed 24-72 hours after onset. According to the characteristics of various perfusion parameters, the perfusion defect area in CTP was divided into group Ⅰ (compensatory phase of cerebral circulation reserve), group Ⅱ (compensatory phase of cerebral metabolism reserve), group Ⅲ (hyperacute phase of cerebral infarction). The region of interest (ROI) in each perfusion defect area and the contralateral mirror perfusion normal area was delineated, and the mean values of regional cerebral blood flow (rCBF), regional cerebral blood volume (rCBV), mean transit time (MTT), and time to peak (TTP) in the ROI were recorded. The perfusion parameters of normal brain tissue were included in group Ⅳ (normal control group). One-way analysis of variance was used to compare the overall differences in CTP parameters measured in each group in each region, and the multiple comparisons were performed to assess statistical differences between the perfusion parameters of groups in all parts of the posterior circulation. The sensitivity, specificity and accuracy of CTP in evaluating the hyperacute phase of cerebral infarction in various parts of the posterior circulation were calculated by using DWI as a standard. Results: A total of 271 cerebral ischemia or cerebral infarction lesions were detected in 184 patients, 107 in group Ⅰ, 75 in group Ⅱ, and 89 in group Ⅲ. There were statistically significant differences in the perfusion parameters of each group and each part of the posterior circulation (P<0.01). The changes of rCBF and MTT in each territory were not significant between group I and group II, but the decrease of rCBF and the increase of MTT in groups Ⅰ and Ⅱ were significantly different from those in group Ⅳ (P<0.05). The rCBF values of all the territories in group Ⅲ decreased significantly, and the differences between groupⅢ and groups Ⅰ, Ⅱ and Ⅳ were statistically significant (P<0.05). The MTT value of group Ⅲ was significantly increased, and the differences between group Ⅲ and groups Ⅰ, Ⅱ and Ⅳ were statistically significant (P<0.05), except for the difference between groups Ⅲ and Ⅱ in the blood supply area of P2 segment of posterior cerebral artery. rCBV values in cerebellum, pons and blood supply area of P1 and P2 segments of the posterior cerebral arteries were not significantly different among group Ⅰ, group Ⅱ, and group Ⅳ, but the rCBV values of group Ⅲ decreased significantly, and the differences with groups Ⅰ, Ⅱ and Ⅳ were statistically significant (P<0.05). The decrease of rCBV and increase of TTP in midbrain and thalamus of group Ⅱ were significantly different from those in group Ⅰ (P<0.05), while the rCBV value and TTP value of group Ⅱ were not significantly different from those of group Ⅲ. The total sensitivity, specificity and accuracy of CTP in the hyperacute phase of cerebral infarction in the posterior circulation were 79.0%, 99.7% and 98.5%, respectively. Conclusions The CTP parameter maps can reflect the pathophysiological changes of the posterior circulation cerebral ischemia and the hyperacute phase of cerebral infarction. CTP has adequate sensitivity and very high specificity and accuracy for the evaluation of posterior circulation cerebral infarction.