It is unknown whether computed tomographic perfusion (CTP) parameters predict ischemic stroke in patients with symptomatic chronic carotid or middle cerebral artery occlusion.A post hoc analysis of medically treated patients enrolled in the CMOSS trial (Carotid or Middle Cerebral Artery Occlusion Surgery Study; REGISTRATION: URL: https://www.clinicaltrials.gov; Unique identifier: NCT01758614), a randomized controlled trial conducted at 13 centers in China between June 2013 and March 2018. It aimed to compare extracranial-intracranial bypass surgery to medical therapy in patients with symptomatic carotid or middle cerebral artery occlusion and hemodynamic insufficiency. CTP-derived mean transit time and relative cerebral blood flow were collected. The primary outcome was defined as ischemic stroke in the territory of the qualifying artery within 2 years after randomization. The predictive value of CTP for the primary outcome was analyzed by a Cox regression model. Receiver operating characteristic curves were used to calculate optimal cutoff values of CTP parameters.All 165 per-protocol patients (median age=53.7 years, 81.2% males) treated with medical treatment alone were analyzed. Sixteen (9.7%) patients suffered the primary outcome during the 2-year follow-up. Cutoff values of mean transit time >6.5 seconds (symptomatic side) and relative cerebral blood flow ≤0.5 were associated with recurrent stroke. In multivariate Cox regression, mean transit time (adjusted hazard ratio, 3.50 [95% CI, 1.19-10.30]; P=0.02) and relative cerebral blood flow (adjusted hazard ratio, 7.36 [95% CI, 2.27-23.85]; P=0.001) were independently associated with the primary outcome.CTP-based hemodynamic parameters are predictive of recurrent ischemic stroke in symptomatic patients with chronic carotid or middle cerebral artery occlusion. CTP could be used in patient selection for stratified secondary prevention of stroke in future studies.URL: https://www.clinicaltrials.gov; Unique identifier: NCT01758614.