Objective: The assessment of aortic stiffness by carotid-femoral pulse wave velocity (PWVcf) is conventionally done in the supine position with uniform mean distending pressure along the aortic trunk. However, humans spend a large part of their time in the upright position, where effects of gravity produce a graded distending pressure, in addition to mean arterial pressure (MAP). This study investigates the effect of posture on PWV independent of changes in MAP in two groups of young and elderly subjects. Methods: PWVcf and MAP were measured during head-up tilt in 4 positions (0°, 30°, 60° and 90°) in 14 young male subjects (25-37 yrs) and in 8 male subjects (47-67 yrs) Transit time for PWVcf was obtained with applanation tonometry (SphygmoCor, Atcor Medical, Sydney) and MAP was obtained with a brachial cuff at heart level for all tilt positions. Pressure due to gravity was determined at the mid-point of the carotid-femoral path in relation to the position of femoral artery (MAPcal). Results: Results for the different tilt angles showed proportional graded changes. There was an average increase in PWVcf of 25% from supine to upright position in the young cohort and 24.6% in the old. When corrected for changes in MAP and heart rate this changed to 26.8% increase in the young group and 22.1% (a reduction of 4.7%) in the older group, with only a 1.2% difference in MAPcal. Conclusions: This study quantifies the age-related hydrostatic effects of gravity on measurements of aortic stiffness as assessed by PWVcf. Postural changes are associated with increases in aortic stiffness due to the additional gravitation effects on arterial distending pressure. This effect is reduced with age, possibly due to reduced aortic distensibility with age as is seen by higher PWVcf.