The objective of this study was to determine the utility of computed tomography perfusion (CTP) scans in diagnosing acute ischemic stroke (AIS). CTP abnormalities in two contiguous supratentorial levels were correlated with diffusion weighted image (DWI) abnormalities on magnetic resonance imaging (the standard for stroke diagnosis) performed within 1 week in 422 suspected acute ischemic strokes. Of 157 AIS confirmed by DWI, 78 showed CTP abnormalities (sensitivity 49.7%). All of these examinations were performed with a mean of 35 min from the initial emergency department neurological examination. Excluding small non-vascular territory strokes left 77 AIS with a total volume of infarcted tissue of more than 5 cc by DWI. Of these, 71 showed CTP abnormalities (sensitivity 92.2%). Of the 265 patients without AIS, none showed CTP abnormalities (specificity 100%). CTP is a rapid, readily available and effective method of diagnosing AIS in clinical practice, particularly the major intracranial vessel strokes that result in a more devastating outcome.