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Arterial stiffness may indicate early vascular changes that predispose to the development of major vascular disease. The repeatability of a variety of indices of arterial stiffness calculated from a standard carotid arterial M-mode ultrasound image was investigated.Twenty-six asymptomatic normal subjects were imaged and had blood pressure recordings on each of two separate occasions at least 1 day apart. Using a computer-assisted method, the maximum and minimum internal diameter and average wall thickness of the right common carotid artery were measured over several cardiac cycles, and the following indices of arterial stiffness and distensibility (compliance) were derived: the pressure-strain elastic modulus (Ep), Young's modulus (E), cross-sectional compliance (CC), and the distensibility coefficient (DC).The repeatability of these measures, expressed as coefficients of variation, was as follows: Ep, 18%; E, 24%; CC, 14%; and DC, 13%. In another group of 20 subjects, the coefficient of variation for repeat examination by different sonographers was Ep, 19%; E, 20%; CC, 14%; and DC, 17% and for the one sonographer using two ultrasound machines was Ep, 13%; E, 13%; CC, 11%; and DC, 13%. These values indicate a moderate level of repeatability. In a univariate analysis each of these indices was significantly related to increasing age (Ep=1.0+12.9×AGE, r=.80; E=314.5 +13.9×AGE, r=.48; CC=22.6−0.26×AGE, r=−.63; DC=64.0−0.65×AGE, r=−.78) but not to wall thickness (all P > .47). Using multiple regression techniques to adjust for age, wall thickness is a significant predictor of distensibility (P=.017), cross-sectional compliance (P < .001), and the pressure-strain elastic modulus (P=.019). Because Young's modulus is calculated from wall thickness, it could not be included in the multivariate analysis.We conclude that estimates of carotid artery distensibility and cross-sectional compliance derived from M-mode ultrasound recordings are moderately repeatable and may provide useful additional end points for trials of atherosclerotic progression.