A small percentage of patients with asymptomatic carotid artery stenosis (ACAS) who are on optimal medical management do go on to develop ischemic stroke or transient ischemic attacks (IS/TIA). Several diagnostic tools have been studied to identify those patients who are at increased risk. However, most of these diagnostic tools are not available for routine clinical use or are resource intensive. We performed a retrospective study to assess the incremental value of external carotid artery stenosis progression (ECASP) along with internal carotid artery stenosis progression (ICASP) in predicting risk of ipsilateral IS/TIA in a cohort of patients with ACAS. We conducted a retrospective analysis of patients with ACAS who had at least two serial duplex ultrasounds (DUS) at our center. A total of 356 patients (712 carotid arteries) were included in the study (mean age 74.7±9 years, 49.2% male) with a mean follow-up of 60.7±32.7 months. In univariate analysis, concurrent progression of ICA and ECA stenosis on the same side arteries was associated with a very significant increased risk of ipsilateral IS/TIA (14.7% vs 4.6%, p<0.001). Also, multivariable regression analysis showed that concurrent ECA/ICA progression was an independent predictor of IS/TIA (OR=3.6, 95% CI 1.64–7.8; p=0.001). ECASP along with ICASP is significantly associated with increased risk of ipsilateral IS/TIA and provides incremental risk stratification over that provided by ICASP alone. The ECA is routinely evaluated in clinical practice, and it could serve as an additional marker for identifying higher risk patients with ACAS.