Background and Purpose- The modified Treatment In Cerebral Ischemia (mTICI) score is the standard method to quantify the degree of reperfusion after endovascular treatment in acute ischemic stroke. In clinical practice, it is commonly assessed by local operators after the procedure. In clinical trials and registries, mTICI is evaluated by an imaging core lab. The aim of this study was to compare operator mTICI with core lab mTICI scores in patients included in the MR CLEAN (Multicenter Randomized Clinical Trial of Endovascular Treatment for Acute Ischemic Stroke in the Netherlands) Registry. Methods- All patients with an intracranial carotid or middle cerebral artery occlusion with anteroposterior and lateral digital subtraction angiography runs were included. Operators determined the mTICI score immediately after endovascular treatment. Core lab neuroradiologists were blinded to clinical characteristics and assessed mTICI scores based on pre- and postintervention digital subtraction angiography. The agreement between operator and core lab mTICI scores and their value in the prediction of outcome (score on modified Rankin Scale at 90 days) was determined. Results- In total, 1130 patients were included. The proportion of agreement between operator and core lab mTICI score was 56% (95% CI, 54%-59%). In 33% (95% CI, 31%-36%), mTICI was overestimated by operators. Operators reported a higher rate of successful reperfusion than the core lab (77% versus 67%; difference 10% [95% CI, 6%-14%]; P<0.001). In 252 (33%) of 763 patients scored as incomplete reperfusion by the core lab (mTICI <3), the local read was mTICI 3. Multivariable logistic regression models containing either core lab scored or operator scored successful reperfusion predicted outcome on the full (C statistic of both models: 0.76) or dichotomized modified Rankin Scale (modified Rankin Scale, 0-2; C statistic of both models: 0.83) equally well. Conclusions- Operators tend to overestimate the degree of reperfusion compared with the core lab although this does not affect the accuracy of outcome prediction.