Purpose To develop a quantitative assessment of collateral perfusion at CT and to investigate its value in the prediction of outcome in patients with acute ischemic stroke (AIS). Materials and Methods This retrospective study reviewed data from consecutive patients with AIS and an occluded M1 segment of the middle cerebral artery who underwent pretreatment perfusion CT between May 2009 and August 2017. The maximum cerebral blood flow (CBF) of collateral vessels (cCBFmax) within the Sylvian fissure was calculated for each patient. Good outcome was defined as a 90-day modified Rankin scale score of 0-2. Multivariable logistic regression analysis was used to determine the relationship between cCBFmax and (a) hemorrhagic transformation and (b) clinical outcome. Results The final analysis included 204 patients (median age, 73 years; interquartile range, 62-80 years; 82 [40.2%] women). Multivariable logistic regression analysis showed that higher cCBFmax was an independent predictor for (a) a lower risk of hemorrhagic transformation (odds ratio [OR], 0.99; 95% confidence interval [CI]: 0.98, 1.00; P = .009) after adjusting for baseline National Institutes of Health Stroke Scale (NIHSS), endovascular thrombectomy, baseline infarct core volume, and recanalization and (b) better outcome (OR, 1.02; 95% CI: 1.01, 1.03; P = .001) after adjusting for age, baseline NIHSS score, endovascular thrombectomy, hypertension, baseline infarct core volume, and recanalization, respectively. Conclusion The measurement of maximum cerebral blood flow of collateral vessels within the Sylvian fissure is a feasible quantitative collateral assessment at perfusion CT. Maximum cerebral blood flow of collateral vessels was associated with clinical outcome in patients with acute ischemic stroke. © RSNA, 2019 Online supplemental material is available for this article.