BACKGROUND Cervical artery dissection is often attributed to an underlying arteriopathy related to a generalized extracellular matrix defect. OBJECTIVE The authors compared the hemodynamic and morphologic properties of the carotid artery, as assessed noninvasively by ultrasonography, in patients with spontaneous internal carotid artery dissection (ICAD) and control subjects. METHODS Twenty-six patients who experienced ICAD more than 6 months before evaluation were compared with 26 controls matched for age, sex, and height. Cases and controls had ultrasound measurement of common carotid artery diameter and diameter change during the cardiac cycle, bulbar and suprabulbar internal carotid artery diameters, and common carotid artery intima-media thickness. The unaffected carotid artery in cases was compared with the carotid artery of the same side in controls. RESULTS Common carotid artery relative diameter change was significantly higher in cases than controls, whereas other measurements were not significantly different between the groups. In multivariate analyses, the highest tertile of common carotid artery relative diameter change was associated with the risk of ICAD (OR, 10.0; 95% CI, 1.8 to 54.2; p = 0.002) CONCLUSIONS An underlying arteriopathy, presumably related to an extracellular matrix defect, may be present in patients with spontaneous ICAD.