To study the changes in peak systolic velocities of the ipsilateral external carotid artery (ECA) following carotid revascularization.All patients randomized to carotid artery stenting (CAS) or carotid endarterectomy (CEA) in the International Carotid Stenting Study (ICSS; ISRCTN25337470) in our center were included. Peak systolic velocities (PSV) were assessed with duplex ultrasound (DUS) at baseline, at 30 days, and at 12 and 24 months after treatment. Our primary outcome measure was the change in blood flow velocities in the ECA (ΔPSVECA). Secondary outcome measure was the prevalence of post interventional ECA occlusion.Of 270 patients enrolled in ICSS at our center, 224 patients (mean age, 68.8 years; 154 males) were included in the present study (116 CAS, 108 CEA). Baseline PSV in the ipsilateral ECA was similar between the groups. Following CAS, PSV gradually increased during follow-up, whereas PSV remained relatively stable after CEA; mean difference of PSV between CAS and CEA: 23 cm/s (95% CI, -5 to 52), 58 cm/s (95% CI, 27-89), and 69 cm/s (95% CI, 31-107) at 30 days, 12 months, and 24 months. One new ECA occlusion occurred after CAS and two after CEA.Blood flow velocities in the ipsilateral ECA increase significantly after CAS but not after CEA. However, this does not lead to a higher rate of ECA occlusion in the first 2 years after revascularization. We conclude that CAS is not inferior to CEA in preserving the ECA as a possible potential collateral pathway for cerebral blood supply within 2 years following revascularization.