Evaluation of the cerebral circulation dynamics in patients with stroke soon after onset is extremely important for planning the optimum treatment. Perfusion computed tomography (CT) was evaluated in 118 patients with stroke within 10 hours of onset in whom initial CT showed no ischemic change. The mean transit time (MTT), cerebral blood flow (CBF), and cerebral blood volume (CBV) perfusion CT maps were visually evaluated in three slices covering the body of the lateral ventricle, the basal ganglia, and the pons, and the ratios of the values in regions of interest (ROIs) in the ischemic lesion and the symmetrical location in the opposite hemisphere were calculated (ROI ratio of regional (r) MTT, rCBF, and rCBV). The location of occlusion was confirmed by angiography performed on the same day in 106 cases and the location of infarction by later magnetic resonance (MR) imaging. MTT maps correctly identified 44 of 46 cases of ischemia in the carotid system, 20 of 29 cases of ischemia in the vertebrobasilar system, and 11 of 35 cases of ischemia in perforator regions. Eight cases could not be identified by perfusion CT, angiography, or MR imaging. The ROI ratios at the upper boundary of infarction (n = 18) were: rCBF 0.574 +/- 0.220 (mean +/- SD) and rCBV 0.972 +/- 0.276, and at the lower boundary of non-infarction (n = 24) were: rCBF 0.504 +/- 0.247 and rCBV 0.815 +/- 0.169; showing a statistical significance of p = 0.348 for CBF and p = 0.026 for CBV (unpaired t-test). The perfusion CT MTT maps correlated well with the angiographical findings for the carotid system, but poorly for the vertebrobasilar system and the perforator regions. A rCBF ratio of 0.5 and rCBV ratio of 0.9 were established for the boundaries of ischemia.