Previous studies have addressed the differences in risk factors and stroke mechanisms between ICAS and ECAS, 2,3 but the results are inconsistent. Whereas some reported that hypertension, diabetes mellitus, and metabolic syndrome (MetS) are more closely associated with ICAS, 4-8 this was not confirmed by others. 9-11 It also remains unclear whether the risk factors and stroke mechanisms differ between anterior and posterior circulation atherosclerosis. 12Previous studies examined a small number of patients at single hospitals. 6,7,11 Most importantly, prospective studies that used detailed examinations, including advanced brain and vascular imaging, are scarce. The purpose of this study was to elucidate the differences in the risk factors and mechanisms of stroke between ICAS and ECAS, and between anterior and posterior circulation atherosclerosis. We developed a Background and Purpose-The aim of this study was to investigate differences in risk factors and stroke mechanisms between intracranial atherosclerosis (ICAS) and extracranial atherosclerosis (ECAS) and between anterior and posterior circulation atherosclerosis. Methods-A multicenter, prospective, Web-based registry was performed on atherosclerotic strokes using diffusionweighted magnetic resonance imaging and magnetic resonance angiography. Stroke mechanisms were categorized as artery-to-artery embolism, in situ thrombo-occlusion, local branch occlusion, or hemodynamic impairment. Results-One-thousand patients were enrolled from 9 university hospitals. Age (odds ratio [OR]