Background: Ischemic stroke causes serious long-term disability and a great number of economic losses. Thrombolytic therapy is used only if the time of stroke onset was <4.5 hours. However, new categories such as wake-up and day un-witnessed strokes, patients unable to tell exact time of last seen well. The importance of study is to use diffusion weighted/Fluid attenuated inversion recovery (DWI/FLAIR) mismatch as a radiological marker which can help to identify patients with lacunar and non-lacunar stroke within 4.5 hours of onset and use it to determine whether patients with unknown onset  stroke  qualify for thrombolytic therapy or not. Patients and methods: prospective cohort study was conducted on 72 patients with known time of symptoms onset, imaged within 24 hours from stroke onset. Patients underwent the admission Computed tomography CT and magnetic resonance scans (DWI and FLAIR only) with time gap was no longer than one hour. The presences of lesions in the neuroradiological modalities were assessed in correlation with the duration of the stroke.  Results: The time from stroke onsetto neuroimaging was significantly shorter with ischemic lesions visible in DWI/FLAIR mismatch group when compared to other modalities. The DWI/FLAIR was characterized by global specificity 100%, sensitivity 91.9%, PPV 100% and NPV 92.1%. It succeeded to diagnose 12 patients with lacunar stroke before 4.5 hours from the stroke onset.  Conclusion: The presence of acute ischemic lesions only in DWI/FLAIR mismatch group can help to identify both lacunar and non-lacunar stroke patients who are within 4.5 hours' time window for intravenous thrombolysis.