Objective: The purpose of this study was to verify transhemispheric diaschisis in the early hours after an ischemic event. Methods: XeCT cerebral blood flow (CBF) studies within 8 h of stroke were studied in 23 patients. Mean CBF was evaluated in the ischemic area, contralateral hemisphere and ipsilateral cerebellum. Results: A severe CBF reduction was found in the ischemic area (mean 9 ± 3 ml/100 g/min). The mean CBF in the unaffected hemisphere (33 ± 10 ml/100 g/min) was 35% less compared to the normal mean value. CBF was decreased in the cerebellum ipsilateral to the stroke (mean 31 ± 12 ml/100 g/min) suggesting a blood flow depression of the whole brain. Conclusions: During the initial hours of cerebral ischemia, the asymptomatic hemisphere demonstrated CBF depression that was part of the global flow reduction.