Based on angiographic and carotid ultrasonographic findings in 60 patients with stroke or other diseases, we studied what factors affected the side-to-side differences of the common carotid artery diameter (CCAD). The side-to-side differences of the CCAD were within 0.7 mm in patients with normal cerebral angiogram, but a difference above 0.7 mm was found in many patients with asymmetry of the circle of Willis or different vasculopathies, such as the carotid artery occlusion, cerebral arteriovenous malformation and aortitis.