Background: Hyperdensity of the middle cerebral artery (MCA) on unenhanced CT is a recognized sign associated with brain's early ischemia. The number of studies which showed a hyperdense posterior cerebral artery (HPCA) sign in posterior circulation infarct is relatively small. We investigated the prevalence of the HPCA sign, correlations with ischemic lesion volume, and stroke risk factors. We also determined the association with prothrombotic and inflammatory markers which have not been studied before. Material/Methods: In the group of 376 patients with a first acute stroke consecutively admitted to Emergency Department, early signs of brain infarction were visible in 221 (58%) cases. Fifty five (25%) subjects had ischemic lesions in the brain supplied by the posterior circulation. We analyzed the unenhanced CT scans, calculated the density of the posterior cerebral arteries, infarct volume, and assessed the relation of the HPCA sign to other factors. Results: The HPCA sign appeared on CT scans of 12 (22%) patients with evidence of the posterior circulation infarct. The density (in Hounsfield units) of the affected PCA was 46.5 comparing to 20.2 of an intact vessel (p<0.0001). The stroke volume was larger when the HPCA sign was observed (medians: 17.6 vs. 4.3 cm3, p=0.02); in multivariate analysis this association was still significant (OR=1.07; 95% CI, 0.99–1.13). The C-reactive protein and fibrinogen levels were significantly higher (p=0.02 for both factors) in patients with the HPCA sign in the univariate analysis. Conclusions: The HPCA may be considered as an additional marker of early brain infarct, especially with large lesion volume.