Background: Perfusion computed tomography (CT) is an evolving technique in the diagnosis of acute stroke. After complex deconvolution algorithms, perfusion color maps-cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT)— are produced, they give visual confirmation of perfusion deficit in ischemic area, but some discrepancies exist regarding this technique in reliability of quantitative detection of tissue viability: penumbra (tissue at risk) that surrounds core (necrosis). Purpose: The purpose of this prospective study was to define brain tissue viability grade on the basis of the perfusion CT parameters in acute stroke patients. Material and Methods: Multimodal CT imaging protocol (unenhanced CT of the brain, CT angiography of head and neck blood vessels, followed by brain perfusion CT and 24 hours follow-up brain CT) was performed. Perfusion deficit was detected first visually, with subsequent manual quantitative and relative measurements in affected and contra-lateral hemisphere in 87 patients. Results: Visual perfusion deficit on perfusion CT images was found in 78 cases (38 female, 40 male; mean age 30 - 84 years): penumbra lesion (n = 49) and core lesion (n = 42). Both lesion types were detected by increased MTT on perfusion CT map, penumbra area showed mean values 202.40% (113% - 345% ± 57.17) and core (41% - 320% ± 64.53) in comparison to contra-lateral hemisphere. CBV mean values in penumbra group were increased up to 113.10% (45% - 276% ± 36.29) and in core decreased till 41.82% (3% - 107% ± 27.09). CBF values were decreased up to 65.63% (31% - 137% ± 22) in penumbra lesion and markedly decreased till 25.94% (4% - 79% ± 17.35) in core. Conclusion: Our study shows that perfusion CT measurements relative threshold values are recommended in definition of penumbra and core lesions in acute stroke patients.