Progressive asymptomatic carotid stenosis identified a subgroup with about twice the risk of ipsilateral stroke compared with those without progression. However, the clinical value of screening for progression simply for selecting patients for carotid procedures is limited because of the low frequency of progression and its relatively low associated stroke rate. The cost effectiveness of screening for change in stenosis severity to better direct current optimal medical treatment needs testing. A number of natural history studies in patients with asymptomatic carotid stenosis have investigated the association between stenosis progression and risk of ipsilateral cerebrovascular events,1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12 and mostly concluded that progression to >80 stenosis in relation to the diameter of the distal internal carotid (North American Symptomatic Carotid Endarterectomy Trial [NASCET] method) was associated with an increased risk of cerebrovascular events. However, these previous studies had significant limitations such as retrospective design, small sample sizes, short duration of follow-up, suboptimal medical treatment, and often stenosis progression detected after ipsilateral stroke or transient ischemic attack was included in predictive testing. Despite the uncertain clinical significance of progressive asymptomatic carotid stenosis, it remains a common reason for repeated carotid duplex imaging and referral for carotid surgery to reduce stroke risk. The Asymptomatic Carotid Stenosis and Risk of Stroke (ACSRS) Study was a prospective international cohort study. The primary objective13 and 14 was to assess the cerebrovascular risk stratification potential of combinations of patients baseline clinical and biochemical characteristics, ultrasound-determined degree of stenosis, and plaque morphology. Particularly important were history of contralateral transient ischemic attack or stroke, stenosis severity at baseline, and plaque texture features (grayscale median [GSM], size of plaque area, size of juxtaluminal black area, and presence of discrete white areas [DWAs]), which could stratify stroke risk from less than 1 per year to more than 10 per year A secondary objective of the ACSRS study was to assess the stroke risk stratification value of stenosis progression or regression using serial (6-monthly) duplex scanning. The aims of the present report were to determine: 1)The incidence of stenosis progression (with or without progression to occlusion) and regression; 2)the association of baseline clinical, biochemical, and plaque features predictive of stenosis progression or regression; 3)the predictive value of changes in the severity of stenosis in terms of ipsilateral cerebral or retinal ischemic (CORI) events including stroke. Associated with this aim, we determined if stenosis progression is a predictor of cerebrovascular events, is independent of baseline stenosis, and investigated any additional predictive value of change in stenosis severity compared with our previously published ?ACSRS Best Baseline Risk Stratification Model for Stroke.?