Background and Purpose- Previous studies suggest that intracranial carotid artery calcification (ICAC) volume might influence the clinical outcome of patients after endovascular treatment (EVT) for acute ischemic stroke. Importantly, ICAC can be subtyped into a medial or intimal pattern that may differentially influence the effect of EVT in patients with acute ischemic stroke. Methods- All 500 patients included in the MR CLEAN (Multicenter Randomized Clinical trial of Endovascular treatment for acute ischemic stroke in the Netherlands) were evaluated. Volume (mm3) and location pattern (tunica intima or tunica media) of ICAC could be determined on baseline noncontrast computed tomography in 344 patients. Functional outcome at 90 days was assessed with the modified Rankin Scale. Next, we investigated the association of ICAC volume and pattern with functional outcome using adjusted ordinal logistic regression models. Effect modification by EVT was assessed with an interaction term between treatment allocation and ICAC aspect. Results- We found evidence for treatment effect modification by ICAC pattern ( P interaction=0.04). Patients with predominantly medial calcification had better functional outcome with EVT than without this treatment (adjusted common odds ratio, 2.32; 95% CI, 1.23-4.39), but we observed no effect of EVT in patients with predominantly intimal calcifications (adjusted common odds ratio, 0.82; 95% CI, 0.40-1.68). We did not find an association of ICAC volume with functional outcome (adjusted common odds ratio per unit increase ICAC volume 1.01 (95% CI, 0.89-1.13). Moreover, we found no evidence for effect modification by ICAC volume ( P interaction=0.61). Conclusions- The benefit of EVT in acute ischemic stroke patients with a medial calcification pattern is larger than the benefit in patients with an intimal calcification pattern. Clinical Trial Registration- URL: http://www.trialregister.nl . Unique identifier: NTR1804. URL: http://www.isrctn.com . Unique identifier: ISRCTN10888758.